Set mypy to require full type annotation for all methods

[gkreitz]

Sets disallow_untyped_defs = True, and adds missing type annotations. This prevents us from forgetting to add type annotations to new functions. Also sets disallow_untyped_decorators = True, not that I expect that we'll add a ton of decorators. The setting check_untyped_defs was dropped, as we now require all functions to be typed.

For code interacting with PlasTeX, we unfortunately end up with a ton of Any, as I don't think we have good documentation on what types we'll be called with.

One actual change in the code - in _build_junk_modifier I made the return value bool via a lambda to get a cleaner type annotation. Aside from that, PR is only type annotations.