Update pip and setuptools in CircleCI

The CircleCI checks have been broken because of outdated setuptools,
this change should fix the issue.

.circleci/config.yml

           command: |
             python3 -m venv venv
             . venv/bin/activate
+            pip install --upgrade pip
+            pip install --upgrade setuptools
             cd ml-agents-envs && pip install -e .
             cd ../ml-agents && pip install -e .
             pip install black pytest-cov==2.6.1 codacy-coverage==1.3.11