Add asyncio support

[christiansandberg]

Adds support for developing asyncio applications with python-can more easily. This can be useful when implementing protocols that handles simultaneous connections to many nodes since you can write synchronous looking code without handling multiple threads and locking mechanisms.

For information I started working a year ago or so on adding native asyncio support for each interface but abandoned it since not all API's support callbacks and the performance would probably be worse than using a thread for reading messages.

[felixdivo]

Uhh, looks really nice!

[codecov]

Codecov Report

Merging #388 into develop will increase coverage by <.01%.
The diff coverage is 63.75%.

@@             Coverage Diff             @@
##           develop     #388      +/-   ##
===========================================
+ Coverage    59.25%   59.26%   +<.01%     
===========================================
  Files           55       55              
  Lines         4261     4328      +67     
===========================================
+ Hits          2525     2565      +40     
- Misses        1736     1763      +27

[felixdivo]

Could you add a test for this on Python 3.4+? This could basically test the same behavior as the normal BufferedReader to cover both, right? Maybe in python-can/test/listener_test.py.

[christiansandberg]

The problem is that testing this will require statements that are not available in Python 2.7 so it won’t even be able to read the file. Not sure if that can be handled by pytest.

[felixdivo]

Hm, I see.

[hardbyte]

Very cool!

I tried it out on python 3.6 and saw messages on a vcan bus, and verified that the new test correctly gets skipped on python 2.7.

Thanks for your patience with this addition