Several mypy related changes,

• Makes mypy a bit less restrictive for variable redefinition to address #16705 (comment) by adding the --allow-redefinition option. For instance, without this flag,

  x = [1, 2, 3]
  x.append(2)
  x = set(x)

  would fail with,

  error: Incompatible types in assignment (expression has type "Set[int]", variable has type "List[int]")
  

  while it doesn't with this flag.
• moves the required mypy flags to setup.cfg to make usage a bit more user friendly.
• update to latest mypy version in CI and pre-commit.