PyO3 0.20.1

This release is a maintenance release to resolve a clippy warning which triggers on function arguments of Py<Self>.

This release also contains a few minor API additions, including optional support for the either and smallvec crates.

Thank you to the following users for the improvements:

@adamreichold
@aldanor
@alex
@daemontus
@davidhewitt
@mejrs
@messense
@neachdainn
@orhun
@suriya-ganesh
@wyfo