Do not use 'old' OpenFF Toolkit API

[mattwthompson]

PR Summary:

A change I made to the toolkit (openforcefield/openff-toolkit#1661) involved changing how attributes are looked up, which made this check

        uses_old_api = hasattr(Topology(), "_topology_molecules")

no longer produce the same behavior as when I wrote it. With 0.14.4, released last night, this line erroneously evaluates to True. There would be a few ways to fix this, but I'm proposing to just remove the dual support. We are no longer supporting the "old" (< 0.11) versions of the toolkit. Note, of course, that this change wouldn't affect any released version of Foyer.

I also noticed tests aren't run in parallel - it's a small and straightforward fix to get CI to use the multiple cores that GHA provides
. I'm lazy.

PR Checklist

---

• Includes appropriate unit test(s)
• Appropriate docstring(s) are added/updated
• Code is (approximately) PEP8 compliant
• Issue(s) raised/addressed?

[codecov]

Codecov Report

Merging #545 (81085c5) into main (d2608b7) will increase coverage by 0.33%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #545      +/-   ##
==========================================
+ Coverage   69.21%   69.55%   +0.33%     
==========================================
  Files          16       16              
  Lines        1673     1662      -11     
==========================================
- Hits         1158     1156       -2     
+ Misses        515      506       -9     

[daico007]

LGTM! Do we want to pin the openff-toolkit version also?

[mattwthompson]

The only constraint is openff-toolkit >=0.11 but that's already in there. The quirk of 0.14.4 wasn't an API break so it should work all the way back to 0.11

[mattwthompson]

Thanks @daico007! Anything I can do to help a release? (If you have a checklist/process in place I'm happy to do as much of it as you are comfortable dumping off)

[daico007]

No problem @mattwthompson. I will put a out a patch release for foyer Monday (I may attempt to fix the current Dockerfile before that but there's a good chance it fill fail again)