Improve poetry install speed by first upgrading pip

.github/workflows/ci.yml

@@ -49,6 +49,7 @@ jobs:
             ${{ runner.os }}-poetry-
       - name: Install dependencies
         run: |
+          poetry run pip install --upgrade pip
           poetry install
       - name: Run tests
         run: |

README.md

@@ -323,6 +323,7 @@ datetime.datetime(2019, 1, 1, 11, 59, 58, 800000, tzinfo=datetime.timezone.utc)
 
 ```sh
 # Get set up with the virtual env & dependencies
+poetry run pip install --upgrade pip
 poetry install
 
 # Activate the poetry environment