Add 3.x to Github CI pytest build

[inducer]

@isuruf CI for this will fail, and it already currently fails:

https://github.com/inducer/pymbolic/runs/3987087967?check_suite_focus=true

for pylint because Github defaults to Py 3.10, but symengine doesn't have 3.10 binary builds. What course of action do you recommend?

• Drop symengine for now
• Do the plumbing to build symengine from source no matter what
• Switch to conda
• Wait for 3.10 binary wheels for symengine (are those in the cards?)

[isuruf]

I'd say drop symengine from 3.x always.

SymEngine wheels are made from conda packages which are probably around 2 weeks away.

[inducer]

Thanks! I've taken it in a "best-effort" direction in fe4e38771200ea61fc92cc8eb669881e5cb51d1f.