Update to a compatible version of pip

Anything over v20.x is incompatible with Python 2.x, so avoid.

Author: gavanderhoorn

docker/Dockerfile

@@ -36,7 +36,7 @@ RUN virtualenv -p python2.7 pre141 \
 
 # setup pre venv
 # use older version of setuptools or things won't build
-RUN /rgtm/pre141/bin/pip --no-python-version-warning install -U pip \
+RUN /rgtm/pre141/bin/pip --no-python-version-warning install -U 'pip<21.0' \
  && /rgtm/pre141/bin/pip --no-python-version-warning install -U wheel setuptools==20.1.1 \
  && /rgtm/pre141/bin/pip --no-python-version-warning install nose==1.3.7 \
  && /rgtm/pre141/bin/pip --no-python-version-warning install \
@@ -52,7 +52,7 @@ RUN /rgtm/pre141/bin/pip --no-python-version-warning install -U pip \
  && rm -rf /root/.cache/pip
 
 # setup post venv
-RUN /rgtm/post141/bin/pip --no-python-version-warning install -U pip \
+RUN /rgtm/post141/bin/pip --no-python-version-warning install -U 'pip<21.0' \
  && /rgtm/post141/bin/pip --no-python-version-warning install -U wheel setuptools==44.0.0 \
  && /rgtm/post141/bin/pip --no-python-version-warning install \
       git+https://github.com/rosin-project/rosdistro_python@rosin_bughunt_0.8.3_test#egg=rosdistro \