deps: upgrade PyO3 to 0.21

[cjrh]

This change upgrades PyO3 to 0.21, which introduces a new API for python object references. The following references have more information:

• Blog post by @davidhewitt : Replacing PyO3's API without breaking everything downstream
• Migration guide for PyO3 0.20 -> 0.21

I don't have a super detailed understanding of the upstream changes in PyO3 0.21, but I played whack-a-mole with the compiler and the tests appear to pass. I've pointed out specific areas in the PR where I am unsure.