Add type stubs.

[hoefling]

This PR adds PEP 561 compatibility to msgpack.

I keep the type stubs from this PR for development with msgpack. The idea is to make them available to everyone by adding them to the distribution. There are no changes to the msgpack code itself, the stub is completely separated from the real code.

If you are against adding type stubs, I'd still be grateful for a review that would improve the stub quality - maybe you have a good advice on how to deal with the remaining Any types.

[codecov-io]

Codecov Report

Merging #404 into master will decrease coverage by 0.11%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #404      +/-   ##
==========================================
- Coverage   88.06%   87.94%   -0.12%     
==========================================
  Files           5        5              
  Lines         846      846              
==========================================
- Hits          745      744       -1     
- Misses        101      102       +1

Impacted Files	Coverage Δ
msgpack/fallback.py	88.81% <0%> (-0.14%)	⬇️

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[methane]

I'm sorry, but I am not an expert of typing and I can not maintain it.
I don't want to merge I can not maintain. So I don't merge this pull request anytime soon.

I will drop Python 2 support completely in next major version (in 2021 or 2022). I will reconsider this after it.

[hoefling]

@methane no worries, I see that someone has created a stubs-only package recently: https://github.com/sbdchd/msgpack-types. So this is not an issue anymore, thus closing.