Enable caching for get_ion_reference_data

[rkingsbury]

Cleanups for #428 to enable caching

[rkingsbury]

I'm having trouble understanding why @lru_cache doesn't play well with mypy here. Adding to my confusion, the code here passes mypy linting on my machine but not in the CI.

All unit tests also pass on my local machine, but in the CI there are several of these errors:

___________________ ERROR collecting tests/test_mprester.py ____________________
tests/test_mprester.py:9: in <module>
    from mp_api.matproj import MPRester
src/mp_api/__init__.py:6: in <module>
    from mp_api.client import MPRester
src/mp_api/client.py:39: in <module>
    class MPRester:
src/mp_api/client.py:582: in MPRester
    def _get_ion_reference_data(self, chemsys: tuple):
/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/functools.py:490: in lru_cache
    raise TypeError('Expected maxsize to be an integer or None')
E   TypeError: Expected maxsize to be an integer or None

So basically I'm confused :) Can anyone advise?

[munrojm]

This appears to be an issue with Python 3.7x and not worth spending time fixing. For now, I would add # type: ignore to the end of lines 575 and 577.

Also, for consistency I would vote to change your type hint references to the ones from the typing module. Namely, dict to Dict and tuple to Tuple.

[rkingsbury]

Thanks @munrojm . Linters pass now, and I updated the type hint refs per your suggestion.

[mkhorton]

Merging now, intend to make some further changes later.