Pyee

- Correct version in docs

- Add .readthedocs.yaml file
- Remove vcversioner dependency from docs build

- Remove test_requires and setup_requires directives from setup.py (closing 82)
- Remove vcversioner from dependencies
- Streamline requirements.txt and environment.yml files
- Update and extend CONTRIBUTING.rst
- CI with GitHub Actions instead of Travis (closing 56)
- Format all code with black
- Switch default branch to `main`
- Add the CHANGELOG to Sphinx docs (closing 51)
- Updated copyright information

- Improve thread safety in base EventEmitter
- Documentation fix in ExecutorEventEmitter

- Update README to reflect new API

- Drop support for Python 2.7
- Remove CompatEventEmitter and rename BaseEventEmitter to EventEmitter
- Create an alias for BaseEventEmitter with a deprecation warning