Update black to 21.5b1

[pyup-bot]

This PR updates black from 21.4b1 to 21.5b1.

Changelog

21.5b1

_Black_

- Refactor `src/black/__init__.py` into many files (2206)

Documentation

- Replaced all remaining references to the
[`master`](https://github.com/psf/black/tree/main) branch with the
[`main`](https://github.com/psf/black/tree/main) branch. Some additional changes in
the source code were also made. (2210)
- Sigificantly reorganized the documentation to make much more sense. Check them out by
heading over to [the stable docs on RTD](https://black.readthedocs.io/en/stable/).
(2174)

21.5b0

_Black_

- Set `--pyi` mode if `--stdin-filename` ends in `.pyi` (2169)
- Stop detecting target version as Python 3.9+ with pre-PEP-614 decorators that are
being called but with no arguments (2182)

_Black-Primer_

- Add `--no-diff` to black-primer to suppress formatting changes (2187)

21.4b2

_Black_

- Fix crash if the user configuration directory is inaccessible. (2158)

- Clarify
[circumstances](https://github.com/psf/black/blob/master/docs/the_black_code_style.md#pragmatism)
in which _Black_ may change the AST (2159)

- Allow `.gitignore` rules to be overridden by specifying `exclude` in `pyproject.toml`
or on the command line. (2170)

_Packaging_

- Install `primer.json` (used by `black-primer` by default) with black. (2154)

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 #225