Port bugfixes and enhancements to CPython (4.8.x).

[jaraco]

I realize this change needs a port to CPython, but so also does 4.6.4 and later. I'm unsure if the changes from 4.7 and 4.8 can be safely backported in CPython, so I'll probably need to introduce those changes and the bugfix changes separately.

Originally posted by @jaraco in #357 (comment)

[jaraco]

I can see a couple of ways to accomplish the above:

1. Cherry-pick the bugfixes into the cpython branch and apply those to CPython, then apply the enhancements in a separate PR.
2. Apply all the changes to the CPython branch, but when doing the backports to older Pythons, back out the enhancement changes.
3. Apply changes to CPython in multiple rounds, in the order they were released here, with enhancements and bugfixes separated (e.g. 4.6.4, 4.7.0, 4.7.1, 4.8.0, 4.8.3).

Now that I look at it, the bugfixes 4.6.4 and 4.7.1 and 4.8.1 are not particularly important to CPython. So it looks like I can merge 4.8.1 into cpython, then merge 4.8.3 and backport that.

[jaraco]

Fixed in bpo-46105 and bpo-44893.