Fix flake8 violations

[sbethur]

• This contribution adheres to CONTRIBUTING.md.

- [ ] I've updated CHANGELOG.md if applicable.

- [ ] I've added tests applicable for this pull request

What does this Pull Request accomplish?

flake8 on TravisCI got updated recently from 3.7.9 to 3.8.1. On the new version, checks for '...'.format(...) have become stricter.

List issues fixed by this Pull Request below, if any.

None

What testing has been done?

CI will run tests.

[codecov]

Codecov Report

Merging #1450 into master will not change coverage.
The diff coverage is 92.30%.

@@           Coverage Diff           @@
##           master    #1450   +/-   ##
=======================================
  Coverage   91.93%   91.93%           
=======================================
  Files          20       20           
  Lines        3657     3657           
=======================================
  Hits         3362     3362           
  Misses        295      295           

Flag	Coverage Δ
#codegenunittests	88.25% <92.30%> (ø)
#nifakeunittests	96.40% <ø> (ø)
#nimodinstunittests	95.37% <ø> (ø)
#nitclkunittests	95.45% <ø> (ø)

Impacted Files	Coverage Δ
build/helper/codegen_helper.py	92.44% <87.50%> (ø)
build/helper/documentation_helper.py	89.42% <100.00%> (ø)

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