Epid: enable (a subset of ?) `mypy --strict` checks

[shadeofblue]

to prevent issues similar to:

• #935

• maybe in a staged way, by enabling one check at a time
• consider not adding all of the checks that constitute --strict

• [ ] --warn-unused-configs <-- requires no changes
• [ ] --disallow-any-generics <-- 86 errors and seems really useful
• [ ] --disallow-subclassing-any <-- 3 errors and we probably don't want this
• [ ] --disallow-untyped-calls <-- 111 errors and we might want it in selected places only
• [ ] --disallow-untyped-defs
• [ ] --disallow-incomplete-defs
• [x] --check-untyped-defs ( #935 )
• [ ] --disallow-untyped-decorators
• [x] --no-implicit-optional ( #960 )
• [ ] --warn-redundant-casts
• [ ] --warn-unused-ignores
• [ ] --warn-return-any
• [ ] --no-implicit-reexport
• [ ] --strict-equality
• [x] @zakaprov says: let's use mypy config file :)
• [x] @johny-b says: consider doing #961 first

Edit:

• Consider --strict with allow-x, allow-y ... vs disallow-x, disallow-y ...