Update black and flake8-bugbear.

Dockerfile.3.6.in

-RUN pip install -U black==19.3b0
+RUN pip install -U black==20.8b1

Dockerfile.3.7.in

-RUN pip install -U black==19.3b0
+RUN pip install -U black==20.8b1

Dockerfile.3.8.in

-RUN pip install -U black==19.3b0
+RUN pip install -U black==20.8b1

Dockerfile.3.9-rc.in

-RUN pip install -U black==19.3b0
+RUN pip install -U black==20.8b1

Dockerfile.alpine.in

@@ -10,5 +10,5 @@ RUN apk add --virtual .build-deps \
     openldap-dev libsass g++ postgresql-client bash \
     libffi-dev
 
-RUN pip install 'flake8-bugbear~=19.8' pipenv tox twine wheel 'xoutil~=2.1' merchise.lint>=0.16 sphinx sphinx-rtd-theme 'python-gitlab~=1.15'
+RUN pip install 'flake8-bugbear~=20.1' pipenv tox twine wheel 'xoutil~=2.1' merchise.lint>=0.16 sphinx sphinx-rtd-theme 'python-gitlab~=1.15'
 RUN pip install git+https://github.com/pycqa/pyflakes@4ed1ba41d1406dbb6c9ffbd369c2f523c2dc48f3

Dockerfile.debian.in

@@ -4,5 +4,5 @@ LABEL maintainer="Merchise Autrement <info@merchise.org>"
 RUN echo 20200515 && apt-get update && \
     apt-get install -y --no-install-recommends build-essential git make gnupg
 
-RUN pip install 'flake8-bugbear~=19.8' pipenv tox twine wheel 'xoutil~=2.1' merchise.lint>=0.16 sphinx sphinx-rtd-theme 'python-gitlab~=1.15'
+RUN pip install 'flake8-bugbear~=20.1' pipenv tox twine wheel 'xoutil~=2.1' merchise.lint>=0.16 sphinx sphinx-rtd-theme 'python-gitlab~=1.15'
 RUN pip install git+https://github.com/pycqa/pyflakes@4ed1ba41d1406dbb6c9ffbd369c2f523c2dc48f3