Switch to flake8 for static code analysis

[jdufresne]

flake8 catches a wider net of mistakes than pycodestyle and is more
commonly used by the larger community. For example, it catches unused
imports, a few of which existed. These have since been removed.

Two "noqa" comments were added. One ignores the _compat.py file as it
has a large amount of Python version specific code. The second is in
utils.py which intentionally does not use an import.

Pull Request check-list

Please make sure to review and check all of these items:

• Does $ tox pass with this change (including linting)?
• Does travis tests pass with this change (enable it first in your forked repo and wait for the travis build to finish)?
• Is the new or changed code fully tested?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?

[codecov-io]

Codecov Report

Merging #1328 into master will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1328      +/-   ##
==========================================
- Coverage   92.88%   92.87%   -0.01%     
==========================================
  Files          20       20              
  Lines        6545     6543       -2     
==========================================
- Hits         6079     6077       -2     
  Misses        466      466              

Impacted Files	Coverage Δ
redis/_compat.py	87.09% <ø> (ø)
redis/client.py	86.26% <ø> (-0.01%)	⬇️
redis/connection.py	82.32% <ø> (-0.03%)	⬇️
redis/utils.py	68.75% <100.00%> (ø)

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[laixintao]

+1 on this

[andymccurdy]

Ya, should have done this sooner. I use flake8 in my dev environment already :)