Pyee

- `EventEmitter` supports pickling
- Development dependencies updated to latest
- Dependency on mock removed in favor of unittest.mock
- Additional type hints so pyright check passes on latest
- Drop 3.7 support

- Add `py.typed` file to `MANIFEST.in` (ensures mypy actually respects the
type annotations)

- Improve type safety of `EventEmitteron`, `EventEmitteradd_listener`
and `EventEmitterlistens_to` by parameterizing the `Handler`
- Minor fixes to documentation

- Add `tests_require` to setup.py, fixing COPR build
- Install as an editable package in `environment.yml` and
`requirements_docs.txt`, fixing Conda workflows and ReadTheDocs
respectively

- Fix regression where `EventEmitterlisteners` began crashing when called
with uninitialized listeners

Compatibility:

- Drop 3.6 support

New features:

- New `EventEmitter.event_names()` method (see PR 96)
- Type annotations and type checking with `pyright`
- Exprimental `pyee.cls` module exposing an `evented` class decorator
and a `on` method decorator (see PR 84)

Moved/deprecated interfaces:

- `pyee.TwistedEventEmitter` -> `pyee.twisted.TwistedEventEmitter`
- `pyee.AsyncIOEventEmitter` -> `pyee.asyncio.AsyncIOEventEmitter`
- `pyee.ExecutorEventEmitter` -> `pyee.executor.ExecutorEventEmitter`
- `pyee.TrioEventEmitter` -> `pyee.trio.TrioEventEmitter`

Removed interfaces:

- `pyee.CompatEventEmitter`

Documentation fixes:

- Add docstring to `BaseEventEmitter`
- Update docstrings to reference `EventEmitter` instead of `BaseEventEmitter`
throughout

Developer Setup & CI:

- Migrated builds from Travis to GitHub Actions
- Refactor developer setup to use a local virtualenv