tests: Consolidate version (2 vs. 3) and platform (CPython vs. PyPy) checks

[EricCousineau-TRI]

Minor updates that may help discussion in #2366

[EricCousineau-TRI]

FWIW I dunno how much I love injecting stuff into pytest, but 🤷, it at least sticks with convention.
Perhaps at some point it would make things more obvious to consolidate these into an explicit utility module, like pybind11_test_utilities or something.

[YannickJadoul]

A few minor remarks, but I like this! :-)
Question remains what to do with @rwgk's tests, since he prefers bytes is str when working with the string/bytes types: #2366 (comment).

[rwgk]

Thanks Eric for taking the initiative!

[YannickJadoul]

Any further suggestions how to continue with this? It would be nice to get this in so new PRs can use this.

[henryiii]

Since we already inject stuff into pytest with pytest_configure, I'd be okay to put this in for now, then do a proper cleanup later. I bet that's what @EricCousineau-TRI meant by convention. There are several bad practices here that need to be fixed, but that's not the focus of this PR. The only change I think we should require now is avoiding PY3 - I can add that, though.

[henryiii]

I've pushed the PY2 only change, and dropped the fail fasts. @EricCousineau-TRI, does this look fine? I'll try to refactor later for pytest best practices, but it shouldn't hold up this PR.

[henryiii]

Sorry, had to rebase & force-push, another check that needs to be converted to PY2 was added, and it was detected by flake8.

[YannickJadoul]

Great, thanks, @henryiii! As far as I'm concerned, this can go in soon enough, then?

[EricCousineau-TRI]

@EricCousineau-TRI, does this look fine?

Yup! Just going through to address the open discussions.

[EricCousineau-TRI]

K, pending CI, this should be good to go! Thank y'all!

[EricCousineau-TRI]

Mac CI going slow :( Looks like there're only 2 nodes available for provisioning... 😿

[YannickJadoul]

Hurray! :-)