Update to the latest version from CPython 3.8

[frenzymadness]

I've managed to update this module to the version we currently have in CPython 3.8. The fix for #55 proposed in #56 is also included.

As I explained in #59, some changes might be a little bit backward-incompatible but not in a sense of compatibility with older Pythons but some parts just get deleted in CPython. I haven't seen anything important.

I had to do some small details in different ways or skip a commit or a part of it, here are some notes from the process:

• https://github.com/python/cpython/commit/58a10967619d0281adf346f227c254f1c3b3f32e Not implemented, uses new backward-incompatible str methods
• https://github.com/python/cpython/commit/e59ec1b05d3e1487ca7754530d3748446c9b7dfd Already implemented here (unused methods removed)
• https://github.com/python/cpython/commit/2430d532e240dea55f0082d1e9bf2e0f3d7505be Implemented partially - we have to keep handcrafted cache because functools.cached_property is available only in Python >=3.8.
• https://github.com/python/cpython/commit/6fa84bd12c4b83bee6a41b989363230d5c03b96c Implemented, just self.hostmask is still an attribute and not cached property because the same reason as above
• https://github.com/python/cpython/commit/dc8ce8ead182de46584cc1ed8a8c51d48240cbd5 Cherry-picked from https://github.com/phihag/ipaddress/pull/56 , fixed formatting

All tests are okay locally (in Docker containers) so they should be okay also in the CI. Reviews and tests appreciated.

[shadchin]

@phihag Can you merge this PR?