Defer pandas imports

[mattwthompson]

Description

Unfortunately the wins won't happen until hgrecco/pint#1492 is finished.

>>> import sys, rdkit, openff.toolkit
>>> "pandas" in sys.modules
False
>>> import openff.units
>>> "pandas" in sys.modules
True

Checklist

• Add tests
• Lint
• Update docstrings

[mattwthompson]

Blocked by hgrecco/pint#1492

[mattwthompson]

This should no longer be needed after Pint 0.21, but will need to test to be sure

[mattwthompson]

Woo!

In [1]: >>> import sys, rdkit, openff.toolkit
   ...: >>> "pandas" in sys.modules
   ...:
Out[1]: False

In [2]: >>> import openff.units
   ...: >>> "pandas" in sys.modules
   ...:
Out[2]: False