Allow pytest 8

[mattwthompson]

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.86%. Comparing base (c303f30) to head (ed686ec).

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

• Skip some tests if unpackaged files not present, make some files path-y-er #1825
• Allow writing molecules to paths #1826