CI: Use pip install --upgrade

This will hopefully install the most recent versions of all packages,
including `pyproject-fmt`. 2.8.0 gets installed while 2.18.1 is
available. However, the former fails on Python 3.9:

thread '<unnamed>' panicked at common/src/create.rs:22:5:

Author: amotl

.github/workflows/tests.yml

@@ -62,8 +62,8 @@ jobs:
     - name: Set up project
       run: |
         # Install package in editable mode.
-        pip install versioningit wheel
-        pip install --editable=.[test,develop]
+        pip install --upgrade versioningit wheel
+        pip install --upgrade --editable='.[test,develop]'
 
         # TODO: This command should write a `package.json`, then invoke `npm install`.
         node-blue setup