Typing analysis and public type aliases

[noirbizarre]

Hello 👋🏼

This pull-requests adds:

• a tox typing task performing type hinting analysis using MyPy
• rename the flake8.yml workflow into lint.yml (consistent with its name)
• add MyPy to the Lint workflow
• exposes some type aliases (so there are not instanciable as expected in https://github.com/jamielennox/requests-mock/issues/92#issuecomment-514890445) for type intended for requests_mock users:
  • requests_mock.Context to use in response callbacks (alias for requests_mock.response._Context)
  • requests_mock.Request to use in response callbacks (alias for requests_mock.request._RequestObjectProxy)
  • requests_mock.Callback[T] to type response callbacks (alias for Callable[[requests_mock.Request, requests_mock.Context], T])
• fixes all mistyping to requests.Request which are in fact request proxies (typed requests_mock.Request)
• fixes all MyPy errors including those from requests_mock.contrib

This pull-requests doesn't introduce any functional change (no .py file touched, only .pyi stubs)