Add proper configurations for flake8 and mypy

[astanziola]

It is tricky to configure mypy and flake8 for this project, for several reason

• Operator overloading means that mypy sees a redefinition of unused functions. This is a problems inherited from plum.
• Use black and flake8? I like the 2 spaces indent, but doesn't conform with PEP, so perhaps it is time to let it go..

[astanziola]

Done