Install mypy via pip instead of apt

unittests/docker/Dockerfile

@@ -7,9 +7,9 @@ RUN apt-get update && \
     apt-get install -y \
       pylint3 python3-pip tox git \
       curl pycodestyle \
-      python3-sphinx python3-mypy
+      python3-sphinx
 ARG COMMIT="dev"
 # TODO Rename to linkahead
 RUN git clone -b dev https://gitlab.indiscale.com/caosdb/src/caosdb-pylib.git linkahead-pylib && \
     cd linkahead-pylib && git checkout $COMMIT && pip3 install .
-RUN pip3 install recommonmark sphinx-rtd-theme
+RUN pip3 install recommonmark sphinx-rtd-theme mypy