Deprecate the verde.test function (#344)

It's not needed and hardly ever used. The same functionality can be
achieved with `pytest --pyargs verde` (testing the installed version).
Will be removed in v2.0.0.

verde/__init__.py

@@ -67,6 +67,8 @@ def test(doctest=True, verbose=True, coverage=False, figures=True):
         failed.
 
     """
+    import warnings
+
     import pytest
 
     package = __name__
@@ -83,4 +85,12 @@ def test(doctest=True, verbose=True, coverage=False, figures=True):
     args.append("--pyargs")
     args.append(package)
     status = pytest.main(args)
+
+    warnings.warn(
+        f"The '{package}.test' function is deprecated since v1.7.0 and "
+        "will be removed in v2.0.0. "
+        f"Use 'pytest --pyargs {package}' to run the tests.",
+        FutureWarning,
+    )
+
     assert status == 0, "Some tests have failed."