Pin openff-units, pint to avoid typing issues

[dwhswenson]

This is the current suggestion in https://github.com/openforcefield/openff-units/issues/69.

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (7cd4f3f) 99.09% compared to head (5252d5a) 99.09%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #206 +/- ## ======================================= Coverage 99.09% 99.09% ======================================= Files 36 36 Lines 1771 1771 ======================================= Hits 1755 1755 Misses 16 16 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Do you have feedback about the report comment? Let us know in this issue.