extra mock should not be needed

[bnavigator]

Trivial change:

Replace https://github.com/crossbario/txaio/blob/8e090c0da42ce57fa909ac97bf31c4a5fbd458c2/test/test_call_later.py#L29

by

from unittest.mock import patch

available in the python stdlib since 3.3

This removes the dependency on the extra PyPI package 'mock', which likely goes away for future python versions.