add static type checks with mypy and PEP-484

Created by: ollieparanoid

Idea from marmistrz in IRC:

Add types for each variable in pmbootstrap source code with PEP-484
Use mypy in Travis CI to do static type checking
Minimum supported python version would need to be increased to 3.5 (currently we require 3.4)
The static types in the existing code base can be added automatically with pyannotate

Edited Jun 24, 2024 by Administrator

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