Remove unused pytest-mock dependency (#814)

Remove the dependency on pytest-mock, as the `mocker` fixture is not
used anywhere in the source code, and all tests pass without it.

Author: mgorny

setup.cfg

@@ -63,7 +63,6 @@ test =
     pre-commit
     pytest-console-scripts
     pytest-cov
-    pytest-mock
     pytest-timeout
     pytest-tornasync
     pytest>=6.0