PyO3 0.16.1

This release contains a couple of fixes for regressions in PyO3 0.16.0, as well as documentation fixes. The hashbrown optional dependency range has been extended to include its new 0.12 version.

Thank you for the following users for the improvements:

@adamreichold
@alex
@birkenfeld
@davidhewitt
@JerzySpendel
@mejrs
@messense
@PanQL