Update to version from Python 3.8

[Lumír 'Frenzy' Balhar]

Hello

I'd really like to see #55 fixed and released via #56 but it might be also possible to update this module to the version currently available in Python 3.8 (for start).

I'd like to be proactive here but I need to know what are your plans with backward compatibility. Are you okay to break backward compatibility to stay close to upstream or not? For example, this commit changes how ipaddress handles leading zeros in IPv4 addresses.

I think I've found all commits we have to backport to update to the version we have in Python 3.8 now. The last commit from upstream I see implemented here is python/cpython@91dc64b#diff-f67a8b5be6bdc48383d2ab907325a4c75ef998fa7cd0bba382ec2efa7e4f00b5
The list of commits: https://github.com/python/cpython/commits/3.8/Lib/ipaddress.py
For example, the first commit in the list to backport (python/cpython@58a1096) won't be backported because it uses str.isascii() available only in 3.7 and newer.

Any help appreciated!