add a type check CI test

[joachimmetz]

• have a look at https://github.com/google/pytype
  • maybe use mypy is seems to be more widely available mypy --strict dfdatetime
  • Provided by python3-mypy on Ubuntu 20.04
• add a CI test to check type consistency
• Drop Python 3.5 support to fully support type annotations https://travis-ci.com/github/log2timeline/dfdatetime/jobs/341925588

[joachimmetz]

First iteration: log2timeline/dfdatetime#157

[joachimmetz]

To do handle imports with mypy https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-type-hints-for-third-party-library

[joachimmetz]

stubgen to generate stubs for dependencies with type annotations https://mypy.readthedocs.io/en/stable/stubgen.html

[joachimmetz]

Unfortunately from __future__ import annotations is available from Python 3.7 not for 3.6