Pyee

- setup_requires vs tests_require now correct
- tests_require updated to pass in tox
- 3.7 testing removed from tox
- 2.7 testing removed from Travis

- Tag license as MIT in setup.py
- Update requirements and environment to pip -e the package

- Support Python 3.8 by attempting to import TimeoutError from
`asyncio.exceptions`
- Add LICENSE to package manifest
- Add trio testing to tox
- Add Python 3.8 to tox
- Fix Python 2.7 in tox

- Some tweaks to the docs

- Added a `TrioEventEmitter` class for intended use with trio
- `AsyncIOEventEmitter` now correctly handles cancellations
- Add a new experimental `pyee.uplift` API for adding new functionality to
existing event emitters

- Added a `BaseEventEmitter` class which is entirely synchronous and
intended for simple use and for subclassing
- Added an `AsyncIOEventEmitter` class for intended use with asyncio
- Added a `TwistedEventEmitter` class for intended use with twisted
- Added an `ExecutorEventEmitter` class which runs events in an executor
- Deprecated `EventEmitter` (use one of the new classes)