Update black to 21.4b0

[pyup-bot]

This PR updates black from 20.8b1 to 21.4b0.

Changelog

21.4b0

_Black_

- Fixed a rare but annoying formatting instability created by the combination of
optional trailing commas inserted by `Black` and optional parentheses looking at
pre-existing "magic" trailing commas. This fixes issue 1629 and all of its many many
duplicates. (2126)

- `Black` now processes one-line docstrings by stripping leading and trailing spaces,
and adding a padding space when needed to break up """". (1740)

- `Black` now cleans up leading non-breaking spaces in comments (2092)

- `Black` now respects `--skip-string-normalization` when normalizing multiline
docstring quotes (1637)

- `Black` no longer removes all empty lines between non-function code and decorators
when formatting typing stubs. Now `Black` enforces a single empty line. (1646)

- `Black` no longer adds an incorrect space after a parenthesized assignment expression
in if/while statements (1655)

- Added `--skip-magic-trailing-comma` / `-C` to avoid using trailing commas as a reason
to split lines (1824)

- fixed a crash when PWD=/ on POSIX (1631)

- fixed "I/O operation on closed file" when using --diff (1664)

- Prevent coloured diff output being interleaved with multiple files (1673)

- Added support for PEP 614 relaxed decorator syntax on python 3.9 (1711)

- Added parsing support for unparenthesized tuples and yield expressions in annotated
assignments (1835)

- use lowercase hex strings (1692)

- added `--extend-exclude` argument (PR 2005)

- speed up caching by avoiding pathlib (1950)

- `--diff` correctly indicates when a file doesn't end in a newline (1662)

- Added `--stdin-filename` argument to allow stdin to respect `--force-exclude` rules
(1780)

- Lines ending with `fmt: skip` will now be not formatted (1800)

- PR 2053: Black no longer relies on typed-ast for Python 3.8 and higher

- PR 2053: Python 2 support is now optional, install with
`python3 -m pip install black[python2]` to maintain support.

- Exclude `venv` directory by default (1683)

- Fixed "Black produced code that is not equivalent to the source" when formatting
Python 2 docstrings (2037)

_Packaging_

- Self-contained native _Black_ binaries are now provided for releases via GitHub
Releases (1743)

Links

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

[coveralls]

Coverage remained the same at 86.667% when pulling 5315fe1 on pyup-update-black-20.8b1-to-21.4b0 into e3b6dee on master.

[pyup-bot]

Closing this in favor of #287