PyO3 0.18.1

This release fixes several bugs identified since PyO3 0.18.0, including a segfault and a Windows-specific issue with the PyIterator type, which would incorrectly accept any Python class regardless of whether it defined __next__ or not.

Some new APIs have been added such as PyErr::write_unraisable(), PyAny::is_ellipsis() and PyDict::update.

Thank you to the following users for the improvements:

@adamreichold
@alexpyattaev
@davidhewitt
@jjerphan
@jmhodges
@mejrs
@messense
@mitsuhiko
@neachdainn
@qbx2
@samuelcolvin