i.e. abstract methods and conditional imports

Some hooks which perform a cleanup action can now be called en masse
with the "--hook-stage=manual" argument passed to "run":

    pre-commit run --hook-stage=manual [...]

This ensures mocks are reset in the event of exceptions being raised,
thus preventing subsequent subTests from failing due to poisoned mocks.

… along with some improvements discovered by doing so.

Client instances are expected to raise RuntimeError when methods that
send commands to the server are called before the client is connected.

* Move the module to avoid name conflicts when test framework adds test
  directory to sys.paths
* Add typing
* Add >=3.8 mocks support

'mocket' doesn't support Unix sockets and at some point a release seems
to have broken the test.

This reverts commit 9581b712.

A large amorphous commit including *pre-commit* QA hooks running
a variety of checks and the changes necessary to pass those checks.