Remove upgrade of pip from CI workflow

The GitHub Actions environment clearly has a working pip pre-installed,
and it's unlikely that this project relies on any extremely new
features.

.github/workflows/ci.yml

@@ -16,6 +16,5 @@ jobs:
       - uses: actions/setup-python@v1
         with:
           python-version: "3.8"
-      - run: python -m pip install --upgrade pip
       - run: python -m pip install --upgrade tox
       - run: tox