Test ChemPy with SymPy 1.10rc2

[bertiewooster]

SymPy 1.10 is in pre-release testing. Suggest we test ChemPy to make sure the SymPy update doesn't break any ChemPy functionality.

You can install this release candidate with

pip install --pre sympy

[jeremyagray]

I'll add this to my list. One of the next chances I get I'll look into this and some other dependency and warning issues I've discovered recently.