Black incompatibility prevents tox from running

[magnusbaeck]

Description

The current version of Black is incompatible with recent versions of Click (see psf/black#2964), resulting in the following runtime error:

black run-test: commands[0] | black --check --diff .
Traceback (most recent call last):
  File "/home/magnusbk/src/eiffel-protocol/.tox/black/bin/black", line 8, in <module>
    sys.exit(patched_main())
  File "/home/magnusbk/src/eiffel-protocol/.tox/black/lib/python3.7/site-packages/black/__init__.py", line 1372, in patched_main
    patch_click()
  File "/home/magnusbk/src/eiffel-protocol/.tox/black/lib/python3.7/site-packages/black/__init__.py", line 1358, in patch_click
    from click import _unicodefun
ImportError: cannot import name '_unicodefun' from 'click' (/home/magnusbk/src/eiffel-protocol/.tox/black/lib/python3.7/site-packages/click/__init__.py)
ERROR: InvocationError for command /home/magnusbk/src/eiffel-protocol/.tox/black/bin/black --check --diff . (exited with code 1)

We can easily fix this by upgrading Black.

Motivation

Obvious.

Exemplification

N/A

Benefits

Passing tests.

Possible Drawbacks

None.