* Fix a few issues with the Python language host
1. Fix an issue where the UNKNOWN sentinel was leaking into user
programs
2. Fix an issue where Protobuf types were leaking into user programs
In fixing this issues I also added a framework for writing tests against
the Python SDK.
* License headers, and adopt a more idiomatic testing pattern
* Additional idiomatic Python
* That's what I get for trying to be fancy (Travis CI python version is very old and does not respect this form)
* CR feedback: use more comprehensions, typo fix
* Break a circular dependency between resource, runtime.resource, and runtime.rpc
* Don't check in .vscode
* CR: sort inputs, rename global variable, add a test for CustomResource serialization
* Remove accidental code duplication
