[style] Fix flake8 warnings.

Trivial style corrections.
4 jobs for style-ci in 29 seconds (queued for 60 minutes and 24 seconds)
Status Job ID Name Coverage
  Test
passed #11192
py.test

00:00:21

failed #11191
py.test

00:00:01

failed #11189
py.test

00:00:02

 
  Style
passed #11190
flake8

00:00:08