feat: add support for Python 3.10

[aucampia]

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Fixes#374 🦕

NOTE: system test fail for alembic, but that is the same as is happening on main branch, see #382 and #373

[tseaver]

Thanks very much for the patch!

[aucampia]

Re-checked nox in samples/snippets:

====================================================================== 2 passed, 6 warnings in 30.16s =======================================================================nox > Session py-3.10 was successful.nox > Running session readmegennox > Session readmegen skipped: This session had no parameters available..nox > Ran multiple sessions:nox > * lint: successnox > * blacken: successnox > * py-3.6: successnox > * py-3.7: successnox > * py-3.8: successnox > * py-3.9: successnox > * py-3.10: successnox > * readmegen: skipped20211229T193658 iwana@iwana-pc00.coop.no:~/sw/d/github.com/iafork/python-bigquery-sqlalchemy/samples/snippets
$ 

[tswast]

Thanks for the contribution!

I'll work on getting the system tests passing so that we can merge this soon.