Bump six from 1.14.0 to 1.15.0

Bumps six from 1.14.0 to 1.15.0.

Changelog

Sourced from six's changelog.

1.15.0

• Pull request #331: Optimize six.ensure_str and six.ensure_binary.

Commits

• c0be881 Advance version to 1.15.0.
• 714f40f Document #331.
• 05c4f51 Optimize ensure_str and ensure_binary. (#331)
• a5bb7aa Fix ensure_text docs. (#326)
• See full diff in compare view