Pytest-cleanslate

This release:
- adds a workaround for OSError exceptions happening when tests close (I think all) file descriptors, including those we use for communication;
- made `reduce` easier to use as a library;

This release improves `cleanslate-reduce`:
- improves performance by first reducing by module, then by test function;
- adds `--pytest-args` to allow passing in extra arguments to pytest;
- adds tqdm-based progress bar;
- fixes bugs handling `--trace`;

This release adds `cleanslate-reduce`, a command line tool to help find where state pollution is happening.

This release:
- resolves incompatibilities with `pytest-asyncio > 0.21.1`;
- fixes bugs handling `-k` (substring expressions) and `-m` (mark expression) filtering;
- fixes bugs handling `--exitfirst` and `--stepwise`;

This release just fixes the version number.

I PyPI, therefore I am!