Message class changes

[felixdivo]

This adds a deprecation warning to can.Message.extended_id as discussed in #424.

Further, this PR

• specifies when specific deprecated members of can.Message will be removed
• changes len(some_message) to return the DLC instead on len(self.data), so the method also works on remote frames
• makes can.Message(..., check=True) to actually raise a ValueError and not an AssertionError on invalid parameters, as was documented previously; and also make it run when assertions are disabled
• adds small improvements and fixes to can.Message, like Unicode errors being avoided
• adds a hypothesis test for can.Message
• adds minor cleanups to test/*

[codecov]

Codecov Report

Merging #497 into develop will increase coverage by 0.73%.
The diff coverage is 76.74%.

@@             Coverage Diff             @@
##           develop     #497      +/-   ##
===========================================
+ Coverage    63.81%   64.54%   +0.73%     
===========================================
  Files           63       63              
  Lines         5626     5646      +20     
===========================================
+ Hits          3590     3644      +54     
+ Misses        2036     2002      -34

[felixdivo]

@hardbyte @christiansandberg Could one of you review this change?

[hardbyte]