Pytest-motor

- Add support for Windows, Debian, Ubuntu 16.04, and Ubuntu 20.04.
- Improve the readme.
- Add section about how `pytest-motor` works.
- Add installation instructions.

- Add MacOS support.
- Change the mongod binary download to be async.
- Change mongod connection from using unix sockets to ports.
- Add a contributing guide.
- Add package limitations to readme.
- Add MIT license.

- Change to async fixtures.
- Fix package URL.
- Refactor internal fixtures.

- Improve the readme.
- Add an example.
- Add a link to pytest.
- Add a link to Motor.

- Add more package classifiers.

- Fix package description.