mavgen_python: Add python3 target with type annotations

[alehed]

This PR is based on #664 and contains 4 commits. It adds type annotation support in way that should not break mavlink for people stuck on Python2. Everyone else should benefit from the type annotations.

[alehed]

Note that the type of file should be

class BinaryWriter(typing.Protocol):
    def write(self, b: bytes) -> int:
        ...

and not Any, but typing.Protocol is a python 3.8 thing and I want to support Python 3.6+ with the type annotations.

[tridge]

notes from dev call:

• need to pay special attention to mavproxy (including MAVExplorer), ardupilot autotest suite (Tools/autotest/autotest.py) and mavros
• need CI (github actions) tests to ensure this works both for py2 and py3
• need a plan for deployment, so existing scripts automatically get the new generator if running on py3
• note that most scripts access pymavlink via mavutil

[vooon]

Mavros doesn't use --lang=Python in it's build, but may use as a weak dependency for console scripts.
I'd suggest leave --lang=Python and decide which implementation to use based on interpreter version. E.g. if major version == 3 => use Python3.

[alehed]

Mavros doesn't use --lang=Python in it's build, but may use as a weak dependency for console scripts. I'd suggest leave --lang=Python and decide which implementation to use based on interpreter version. E.g. if major version == 3 => use Python3.

In this PR if you specify "Python" or nothing (the default is "Python") it will only generate the type annotations if you are running it with python >= python3.6

[tridge]

getting this with py2 build:
python2 setup.py build install --user

  File "build/bdist.linux-x86_64/egg/pymavlink/dialects/v10/ASLUAV.py", line 67
    def __init__(self, buf: Optional[Sequence[int]] = None) -> None:
                          ^
SyntaxError: invalid syntax

byte-compiling build/bdist.linux-x86_64/egg/pymavlink/dialects/v10/minimal.py to minimal.pyc
  File "build/bdist.linux-x86_64/egg/pymavlink/dialects/v10/minimal.py", line 67
    def __init__(self, buf: Optional[Sequence[int]] = None) -> None:
                          ^
SyntaxError: invalid syntax

[alehed]

getting this with py2 build:
python2 setup.py build install --user

While these errors are annoying, I don't see anything short of disabling bytecode compiling for those versions that fixes these warnings. Note that like this all files that can be byte-compiled, are byte-compiled.

[alehed]

I had to revert the last commit that turned off byte-compilation. It will not create an egg directory if you disable byte compilation, which is what is needed by most places. The syntax errors it shows when building the package using python2 and 3.5 are quite harmless. Also note that we build the package using 3.9, so they won't happen during the official build step.