add type hints to all of the code

In order to use type hint syntax, we need to bump the minimum Python
version to 3.7 and some of the features such as Literal and Protocol are
opted in when a sufficiently recent Python is available.

This does not make all of the code pass type checking with mypy. A
number of typing issues remain, but the output of mypy becomes something
one can read through.

In adding type hints, a lot of epydoc @type annotations are removed as
redundant. This update also adopts black-style line breaking.

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