Convert ctx from an ABC to a mypystub

Required a bunch of renaming from ctx.Ctx to ctx.Context (its real name)

Makes the docs actually show docstrings

Also apparently mypy doesn't like "from typing_extensions import Protocol" and the fact that it didn't complain so far means that we're probably not type checking everything.

docs/api/ctx.rst

 ``ctx`` module
 ==============
 
-.. automodule:: st3m.ui.ctx
+.. automodule:: ctx
    :members:
    :undoc-members:
+   :member-order: groupwise