Release 4.10.0 planning #337

We have enough changes to justify a new feature release, as well as some fallout from recent CPython releases (#335 (comment)).

My plan now is:

• Cut 4.10.0rc1 on Sunday (Feb. 18)
• Assuming no new issues are raised, cut 4.10.0 the next Sunday (Feb. 25)

Hopefully this gives enough time to get the implementation of PEP 728 in (#329).