Tests fail when no SciPy installed

[feefladder]

What happened:
Tests failed, also reported in #5564 and relatd to #5559
What you expected to happen:

Minimal Complete Verifiable Example:

cd xarray
pip install -e .
pip install h5netcdf pytest-xdist netcdf4
py.test -n 4

======================================================= FAILURES =======================================================
_______________________________________ TestH5NetCDFFileObject.test_open_fileobj _______________________________________
[gw3] linux -- Python 3.9.5 /home/joeperdefloep/miniconda3/envs/xr-dev/bin/python3.9

self = <xarray.tests.test_backends.TestH5NetCDFFileObject object at 0x7f051a6b59a0>

    def test_open_fileobj(self):
        # open in-memory datasets instead of local file paths
        expected = create_test_data().drop_vars("dim3")
        expected.attrs["foo"] = "bar"
        with create_tmp_file() as tmp_file:
            expected.to_netcdf(tmp_file, engine="h5netcdf")

            with open(tmp_file, "rb") as f:
                with open_dataset(f, engine="h5netcdf") as actual:
                    assert_identical(expected, actual)

                f.seek(0)
                with open_dataset(f) as actual:
                    assert_identical(expected, actual)

                f.seek(0)
                with BytesIO(f.read()) as bio:
                    with open_dataset(bio, engine="h5netcdf") as actual:
                        assert_identical(expected, actual)

                f.seek(0)
                with pytest.raises(TypeError, match="not a valid NetCDF 3"):
>                   open_dataset(f, engine="scipy")

/mnt/e/Git/xarray/xarray/tests/test_backends.py:2887:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
/mnt/e/Git/xarray/xarray/backends/api.py:483: in open_dataset
    backend = plugins.get_backend(engine)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

engine = 'scipy'

    def get_backend(engine):
        """Select open_dataset method based on current engine."""
        if isinstance(engine, str):
            engines = list_engines()
            if engine not in engines:
>               raise ValueError(
                    f"unrecognized engine {engine} must be one of: {list(engines)}"
                )
E               ValueError: unrecognized engine scipy must be one of: ['netcdf4', 'h5netcdf', 'store']

/mnt/e/Git/xarray/xarray/backends/plugins.py:156: ValueError
______________________________________________________ test_3641 _______________________________________________________
[gw0] linux -- Python 3.9.5 /home/joeperdefloep/miniconda3/envs/xr-dev/bin/python3.9

    @requires_cftime
    def test_3641():
        times = xr.cftime_range("0001", periods=3, freq="500Y")
        da = xr.DataArray(range(3), dims=["time"], coords=[times])
>       da.interp(time=["0002-05-01"])

/mnt/e/Git/xarray/xarray/tests/test_interp.py:733:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
/mnt/e/Git/xarray/xarray/core/dataarray.py:1687: in interp
    ds = self._to_temp_dataset().interp(
/mnt/e/Git/xarray/xarray/core/dataset.py:3146: in interp
    variables[name] = missing.interp(var, var_indexers, method, **kwargs)
/mnt/e/Git/xarray/xarray/core/missing.py:633: in interp
    interped = interp_func(
/mnt/e/Git/xarray/xarray/core/missing.py:752: in interp_func
    return _interpnd(var, x, new_x, func, kwargs)
/mnt/e/Git/xarray/xarray/core/missing.py:770: in _interpnd
    return _interp1d(var, x, new_x, func, kwargs)
/mnt/e/Git/xarray/xarray/core/missing.py:758: in _interp1d
    rslt = func(x, var, assume_sorted=True, **kwargs)(np.ravel(new_x))
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = <[AttributeError("'ScipyInterpolator' object has no attribute 'method'") raised in repr()] ScipyInterpolator object at 0x7f1aa547fc40>
xi = <xarray.IndexVariable 'time' (time: 2)>
array([0.00000e+00, 1.57788e+19]), yi = array([0, 1]), method = 'linear'
fill_value = None, assume_sorted = True, copy = False, bounds_error = False, order = None, kwargs = {}

    def __init__(
        self,
        xi,
        yi,
        method=None,
        fill_value=None,
        assume_sorted=True,
        copy=False,
        bounds_error=False,
        order=None,
        **kwargs,
    ):
>       from scipy.interpolate import interp1d
E       ModuleNotFoundError: No module named 'scipy'

/mnt/e/Git/xarray/xarray/core/missing.py:129: ModuleNotFoundError
=================================================== warnings summary ===================================================
xarray/tests/test_dataarray.py::TestReduce1D::test_min[x3-5-2-1]
xarray/tests/test_dataarray.py::TestReduce1D::test_max[x3-5-2-1]
xarray/tests/test_dataarray.py::TestReduce2D::test_min[x2-minindex2-maxindex2-nanindex2]
xarray/tests/test_dataarray.py::TestReduce2D::test_max[x2-minindex2-maxindex2-nanindex2]
  /home/joeperdefloep/miniconda3/envs/xr-dev/lib/python3.9/site-packages/numpy/core/fromnumeric.py:86: RuntimeWarning: invalid value encountered in reduce
    return ufunc.reduce(obj, axis, dtype, out, **passkwargs)

-- Docs: https://docs.pytest.org/en/stable/warnings.html
=============================================== short test summary info ================================================
FAILED xarray/tests/test_backends.py::TestH5NetCDFFileObject::test_open_fileobj - ValueError: unrecognized engine sci...
FAILED xarray/tests/test_interp.py::test_3641 - ModuleNotFoundError: No module named 'scipy'
============= 2 failed, 8243 passed, 3776 skipped, 29 xfailed, 26 xpassed, 4 warnings in 229.01s (0:03:49) =============

Anything else we need to know?:

Environment:

Output of xr.show_versions()

INSTALLED VERSIONS

commit: a874739
python: 3.9.5 | packaged by conda-forge | (default, Jun 19 2021, 00:32:32)
[GCC 9.3.0]
python-bits: 64
OS: Linux
OS-release: 4.19.128-microsoft-standard
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: C.UTF-8
LOCALE: ('en_US', 'UTF-8')
libhdf5: 1.12.0
libnetcdf: 4.7.4

xarray: 0.18.2.dev69+gc472f8a4
pandas: 1.2.5
numpy: 1.21.0
scipy: None
netCDF4: 1.5.7
pydap: None
h5netcdf: 0.11.0
h5py: 3.3.0
Nio: None
zarr: None
cftime: 1.5.0
nc_time_axis: None
PseudoNetCDF: None
rasterio: None
cfgrib: None
iris: None
bottleneck: None
dask: None
distributed: None
matplotlib: None
cartopy: None
seaborn: None
numbagg: None
pint: None
setuptools: 49.6.0.post20210108
pip: 21.1.3
conda: None
pytest: 6.2.4
IPython: None
sphinx: None