Allow pytest 8

[mattwthompson]

• [ ] Tag issue being addressed
• [ ] Add tests
• [ ] Update docstrings/documentation, if applicable
• [ ] Lint codebase
• [ ] Update changelog

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 93.75%. Comparing base (fdf3832) to head (c210bda). Report is 5 commits behind head on main.

:exclamation: Current head c210bda differs from pull request most recent head 4772e11

Please upload reports for the commit 4772e11 to get more accurate results.

Additional details and impacted files

[mattwthompson]

Ha, I've gotten this error before but in a position where it didn't need to be fixed

=========================== short test summary info ============================
FAILED openff/toolkit/_tests/test_forcefield.py::TestForceField::test_load_bad_filelike_object - TypeError: argument of type 'NoneType' is not iterable
FAILED openff/toolkit/_tests/test_forcefield.py::TestForceField::test_do_not_load_in_child_dir - TypeError: argument of type 'NoneType' is not iterable
FAILED openff/toolkit/_tests/test_forcefield.py::TestForceField::test_load_bad_bytes - TypeError: argument of type 'NoneType' is not iterable
FAILED openff/toolkit/_tests/test_forcefield.py::TestForceField::test_load_bad_string - TypeError: argument of type 'NoneType' is not iterable

[mattwthompson]

Splitting off some more discrete ideas which might be nice to have in the installed package before this works nicely, starting with

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