Fix CI for Python 2

Black dropped Python 2 support in 22.1.0. Unfortunately we can't pin
the version as <22.1.0 becasue in the past Black was versioned using
just main.major, not main.major.minor, and they were only releasing beta
versions, hence the 'b' suffix.

Pluggy 1.0.0 dropped Python 2 support.

It seams JJB should drop support for Python 2 too. Otherwise it will
be harder and harder to maintain dependencies...

Change-Id: I13b82a02bb50ab46183ff04aae521b44e475f743

.pre-commit-config.yaml

@@ -21,4 +21,5 @@ repos:
         additional_dependencies:
           - flake8>=3.7.8
           - flake8-black
+          - black==21.12b0 # the last version that supports Python 2
         language_version: python3

test-requirements.txt

@@ -12,3 +12,4 @@ stestr>=2.0.0,!=3.0.0 # Apache-2.0/BSD
 tox>=2.9.1 # MIT
 mock>=2.0; python_version < '3.0'  # BSD
 sphinxcontrib-programoutput
+pluggy<1.0.0 # the last version that supports Python 2