Improvements and fixes

* Add more type hints, enable mypy as part of testing
* Raise h2 exceptions when trying to write a closed stream or connection
* Explicitly create streams when appropriate, rather than all the time
* Fix crash when logging window for closed stream
* Update multidict version

Never index :path header

Add .closed property to HTTP2Connection; more debug logging

Check window on stream creation and set open event accordingly

Simplify default SSL context to make it work in Python Docker images

Add Response class

Add LICENSE.txt