Experiment: drop pip version locking in CI

Just run with whatever `setup-python` gave us and see where that gets
us...

Author: matthewhughes934

.github/workflows/test.yml

@@ -57,11 +57,6 @@ jobs:
         with:
           python-version: ${{ matrix.python-version }}
 
-      - name: Upgrade pip
-        run: |
-          pip install --constraint=.github/workflows/constraints.txt pip
-          pip --version
-
       - name: Install Poetry
         run: |
           pip install --constraint=.github/workflows/poetry-constraints.txt poetry