Update black to 21.6b0

[pyup-bot]

This PR updates black from 21.5b2 to 21.6b0.

Changelog

21.6b0

_Black_

- Fix failure caused by `fmt: skip` and indentation (2281)
- Account for += assignment when deciding whether to split string (2312)
- Correct max string length calculation when there are string operators (2292)
- Fixed option usage when using the `--code` flag (2259)
- Do not call `uvloop.install()` when _Black_ is used as a library (2303)
- Added `--required-version` option to require a specific version to be running (2300)
- Fix incorrect custom breakpoint indices when string group contains fake f-strings
(2311)
- Fix regression where `R` prefixes would be lowercased for docstrings (2285)
- Fix handling of named escapes (`\N{...}`) when `--experimental-string-processing` is
used (2319)

Links

• PyPI: https://pypi.org/project/black
• Changelog: https://pyup.io/changelogs/black/
• Repo: https://github.com/psf/black

[pyup-bot]

Closing this in favor of #381