Fixed flake8 warnings

[u6768]

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

• Fixed minor flake8 warnings
• Flake8 is configured in tox.ini but it is not executed by travis ci. So the CI job is fine but local tox run will fail because of such flake8 warnings

[codecov-io]

Codecov Report

Merging #74 into master will not change coverage. The diff coverage is 100%.

@@          Coverage Diff           @@
##           master     #74   +/-   ##
======================================
  Coverage    99.3%   99.3%           
======================================
  Files          10      10           
  Lines         289     289           
======================================
  Hits          287     287           
  Misses          2       2

Impacted Files	Coverage Δ
cfn_flip/__init__.py	100% <ø> (ø)	:arrow_up:
cfn_tools/yaml_dumper.py	100% <100%> (ø)	:arrow_up:

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[stilvoid]

Perfect, thanks!

[u6768]

Thanks stilvoid for reviewing. Just deleted the branch and it is all set.