Mypy-protobuf

- Use `entry_points:console_scripts` to support long paths to the python interpreter

- Update to newer mypy version - including minor changes to typeshed

- Absolute path to necessary python
- Add forward reference string literal support for enums
- Alias builtin types to avoid collision with field name

- Add `class` to set of python keywords

- Add `Message.DESCRIPTOR`

Older changelogs not available. Check git log if you need them!

test_negative/negative.py:26: error: Argument 1 to "CopyFrom" of "Message" has incompatible type "str"; expected "Simple1"


- Update generated EnumTypeWrapper to be instances of EnumTypeWrapper - for more consistency
with generated python code. Most caller code should not require mypy type changes. Egh
`ProtoEnum.Value('first')` should work either way.