TST: move .str-test to strings.py & parametrize it; precursor to #23582

[h-vetinari]

• precursor to TST: add method/dtype coverage to str-accessor; precursor to #23167 #23582 and API: Series.str-accessor infers dtype (and Index.str does not raise on all-NA) #23167
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff

In #23582, I'm testing all string methods on all inferred dtypes (which uncovered lots of bugs that will be fixed by #23167 and follow-ups).

@jreback required #23582 to be broken up further, so here I'm moving one existing test that effectively uses the same idea to the (IMO) correct module (it's about testing the .str-accessor after all), where the any_string_method fixture can then be used for both the test I'm moving in this PR, and the tests I'm introducing in #23582.

It makes sense to have this fixture that's directly related to .str in the appropriate module, and not pollute the general pandas/conftest.py with it, that's mainly why I'm moving the test.

[pep8speaks]

Hello @h-vetinari! Thanks for submitting the PR.

• There are no PEP8 issues in the file pandas/tests/series/test_api.py !

• In the file pandas/tests/test_strings.py, following are the PEP8 issues :

Line 30:12: E241 multiple spaces after ','
Line 30:26: E241 multiple spaces after ','
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Line 56:35: E241 multiple spaces after ','
Line 57:14: E241 multiple spaces after ','
Line 57:30: E241 multiple spaces after ','
Line 58:15: E241 multiple spaces after ','
Line 58:29: E241 multiple spaces after ','
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Line 68:36: E241 multiple spaces after ','
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Line 69:28: E241 multiple spaces after ','
Line 70:14: E241 multiple spaces after ','
Line 70:29: E241 multiple spaces after ','

[h-vetinari]

Failure is rank segfault...

[jreback]

thanks @h-vetinari

[h-vetinari]

@jreback Thanks!
I think I'll have a celebratory drink tonight for my first PR that went straight through without having to incorporate review feedback. ;-)