Bump flake8 from 3.8.4 to 3.9.0

Created by: dependabot-preview[bot]

Bumps flake8 from 3.8.4 to 3.9.0.

Commits

• 434c108 Release 3.9.0
• 7c96c1e Merge branch 'pycodestyle_2_7_0' into 'master'
• 40aba49 upgrade pycodestyle to 2.7.0
• ac9c66e Add indent-size option
• fd71b86 Merge branch 'pyflakes_3_3' into 'master'
• 1eccf9a upgrade pyflakes to 2.3.x
• 6de8252 Merge branch 'crlf_stdin' into 'master'
• 0bf8d2a ensure crlf line endings of stdin are handled properly
• ff433b2 Merge branch 'fix_linters' into 'master'
• 14857c9 fix test which was not testing anything
• Additional commits viewable in compare view

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