Add mypy integration

[uriyyo]

Add mypy integration.

Drop python 3.5 and 3.6 versions cause they reach EOL.

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Fixes #444

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[msdrigg]

I'd like to see this as well

[stianjensen]

This would be great! Not sure if the library has any active maintainers anymore, though 😕

[AAraKKe]

Is this going to be merged or planed? It seems it has been over a year since there has been any update on the library. It has become almost standard to do type check and using this library means a bunch of type ignroes need to be added.