Make a code check which bans imports of fixtures in tests

[MarcoGorelli]

xref #30894 (comment)

Example: writing

from pandas.conftest import ALL_EA_INT_DTYPES

in a test file should be disallowed because there exists the fixture any_nullable_int_dtype

[SaturnFromTitan]

take

[jorisvandenbossche]

There are some useful things that are imported from conftest.py that cannot be converted to a fixture. So "fixing" this should first discuss what to do instead (or where to move those) ?

[SaturnFromTitan]

Good point @jorisvandenbossche I'll check the current violations and make a proposal here.

[SaturnFromTitan]

Alright, so there are basically three types of cases:

1. indices_dict is imported to create a more specific fixture. I submitted two PRs to resolve these cases as they can easily be replaced with using indices
2. _get_cython_table_params is imported. I suggest moving it to pandas._testing. It's not used for to create fixtures anyways, so it doesn't really make sense to have it in conftest.py
3. Any of these dtype constants are imported. Not sure what to do here... We have to move them somewhere else, but I don't know what's a good location. Maybe create a pandas._dtypes.py?

Wdyt @jreback @simonjayhawkins @jorisvandenbossche?

[simonjayhawkins]

for 1 the two PRs lgtm.

agree with 2.

for 3 could have the constants in a separate module as suggested (like token.py — Constants used with Python parse trees) and then import into testing.py (like tokenize.py — Tokenizer for Python source - All constants from the token module are also exported from tokenize) and then we could do say tm.UNSIGNED_INT_DTYPES