MyPy Error Codes

[WillAyd]

mypy 0.73 came with support for error codes:

http://mypy-lang.blogspot.com/2019/09/mypy-730-released.html

I think we should add these to CI and update docs to require these going forward for any type: ignore statements

@simonjayhawkins

[simonjayhawkins]

i did this in 32f1780 but causes F821 errors and needs to be silenced.

reverted after a couple of weeks in 34bf08c as frustrating not having F821 linting in the ide.

so -1 and revisit once flake8 supports mypy errors codes