more aggressive typing

[JelleZijlstra]

After upgrading async_timeout, I got some errors because I am running mypy with some stricter options (e.g., --no-implicit-optional, --disallow-any-generics). There's related discussion in python/mypy#5260, but in general I believe it's a good practice for libraries to be mypy-clean under strict settings, so that users can use those settings safely.

This PR adds a mypy.ini file with a number of stricter options and fixes the issues found in current async_timeout code.

Also, thanks for being so quick in adding typing and py.typed to your package!

[codecov]

Codecov Report

Merging #48 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #48   +/-   ##
=======================================
  Coverage   94.02%   94.02%           
=======================================
  Files           1        1           
  Lines          67       67           
  Branches       11       11           
=======================================
  Hits           63       63           
  Misses          2        2           
  Partials        2        2

Impacted Files	Coverage Δ
async_timeout/__init__.py	94.02% <100%> (ø)	⬆️

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[asvetlov]

Thank you for the PR.
Sorry for late response, I have very limited spare time now.
Please address my comments (or describe why I'm wrong).