Add type hints and update packaging

[J-M0]

This is kind of a big PR that does a bunch to add type hints to netifaces:

• Updated to latest packaging standards
• Reorganized modules to support type hints
• Added __init__.pyi with type hints
• Updated docstrings to make their view in help() a little nicer

I know you mentioned in #39 that not all constants are available for all platforms. If you know of some conditions to gate constants to certain platforms I can easily add them.

Also, if these changes are too much at once, I can easily break this up into smaller PRs.