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.

This seems reasonable for Unix datagram sockets. It is probably beyond
what is necessary, but 1kB is definitely not enough when pulling BSS
information.