Adds `from __future__ import annotations` to the top of every module,
right below the module's docstring. Replaces any usages of t.List,
t.Dict, t.Set, t.Tuple, and t.Type with their built-in equivalents:
list, dict, set, tuple, and type. Ensures that make test still passes
under Python 3.7, 3.8 and 3.9.
Carlos Muniz