Add typing_extensions to py3.7 deps for Literal

[khaeru]

#44 added use of Literal, which is only available in Python ≥3.8. This raises ImportError, for instance here:

__________ ERROR collecting ixmp/tests/reporting/test_computations.py __________
ImportError while importing test module '/home/runner/work/ixmp/ixmp/ixmp/tests/reporting/test_computations.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
/opt/hostedtoolcache/Python/3.7.17/x64/lib/python3.7/importlib/__init__.py:127: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
ixmp/tests/reporting/test_computations.py:6: in <module>
    import pyam
/opt/hostedtoolcache/Python/3.7.17/x64/lib/python3.7/site-packages/pyam/__init__.py:12: in <module>
    from pyam.core import *
/opt/hostedtoolcache/Python/3.7.17/x64/lib/python3.7/site-packages/pyam/core.py:59: in <module>
    from pyam._ops import _op_data
/opt/hostedtoolcache/Python/3.7.17/x64/lib/python3.7/site-packages/pyam/_ops.py:5: in <module>
    from iam_units import registry
/opt/hostedtoolcache/Python/3.7.17/x64/lib/python3.7/site-packages/iam_units/__init__.py:9: in <module>
    from .currency import configure_currency
/opt/hostedtoolcache/Python/3.7.17/x64/lib/python3.7/site-packages/iam_units/currency.py:6: in <module>
    from typing import Literal, Union
E   ImportError: cannot import name 'Literal' from 'typing' (/opt/hostedtoolcache/Python/3.7.17/x64/lib/python3.7/typing.py)

This commit imports the same from typing_extensions for Python 3.7.

Note that Python 3.7 reached EOL as of 2023-06-06, and we should probably drop support for it soon. This preserves support in the interim.

[khaeru]

Here is a successful CI run with Python 3.7, from a temporary commit that I'll now remove.

[danielhuppmann]

FYI, pyam 2.0 to be released thisxweek will drop support for all versions <3.10 - so I suggest to also remove all versions rather than adding clutter for legacy-support.

[khaeru]

I suggest to also remove all versions rather than adding clutter for legacy-support.

iam-units is needed by message_data and the stack underneath it which support all actively maintained versions of Python (so that will include 3.8 until about 2024-10; 3.9 until 2025-10, etc.). But good to know that 3.7 isn't needed for use via pyam—that means I can come back and drop this 3.7 compat item once we finish removing CI against Python 3.7 in those packages.