Bump flake8 from 3.7.9 to 3.8.3

Bumps flake8 from 3.7.9 to 3.8.3.

Commits

• 181bb46 Release 3.8.3
• 3d68da9 Merge branch 'doctests-flag-desc' into 'master'
• e817c63 Help clarify the option behaviour
• b6d3fca Merge branch 'issues/665' into 'master'
• 9b8f908 fix JobsArgument --help output
• 94304de Merge branch 'issue-662' into 'master'
• a68d4d0 processor: Catch SyntaxError also when generating tokens for a file
• 4071645 Release 3.8.2
• b9fe4d6 Merge branch 'extend_exclude_is_files' into 'master'
• 31c2f9f treat --extend-exclude as a file list
• Additional commits viewable in compare view