Remove python 2, python < 3.6, fix up, add flake8

[stuartarchibald]

This PR:

• Removes Python 2 specific code.
• Removes Python <3.6 specific code.
• Removes build/test infrastructure relating to the above.
• Adds build/test infrastructure for Python 3.8
• Adds flake8 checks to the source and enforces this in CI.
• Fixes up the code base to be flake8 compliant.
• Remove the AUTHORS file, this information is in git and the CHANGE_LOG
• Fixes up the documentation (to some degree, LLVM 9 changes pending).

[sklam]

LGTM

[sklam]

/AzurePipelines run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[stuartarchibald]

Anticipate CI fails will need #554 merging, and then a rerun.

[stuartarchibald]

/AzurePipelines run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[stuartarchibald]

CI passed and @sklam approves. Marking for merge.

[ARF1]

@stuartarchibald @sklam Bumping to python <= 3.6 did not take for the AppVeyor:

C:\projects\llvmlite>conda create -n testenv -q -y python=3.8 cmake 
Fetching package metadata .......
Solving package specifications: .
Error: Package missing in current win-64 channels: 
  - python 3.8*

The result is that the AppVeyor CI build falls back to python 3.5 which means that any PR that uses python 3.6 features fails.

Would it be acceptable to set python to 3.6 instead of python 3.8 for AppVeyor? If yes, #565 does this.