Pyperplan

* Add options to API for removal of static facts from init, and removal of irrelevant operators (Blai Bonet, 14).

* Install code as single package (6, 7, 10).
* Allow executing `python -m pyperplan`.
* Bump minimum Python version to 3.6 (5).
* Run continuous integration tests for Python 3.9 (11).
* Revise documentation.

* Test minisat with GitHub actions.

* Modernize code with pyupgrade.
* Enable all tests.
* Remove unused code.
* Order imports.

* Test code with tox.
* Enable GitHub actions continuous integration testing.
* Convert docs to Markdown.
* Remove bundled argparse copy.
* Format code with black.

* First official release.