Ported setup.py to pyproject.toml (#2555)

* Ported setup.py to pyproject.toml, refs #2553

* Make fixtures tests less flaky

The in-memory fixtures table was being shared between different
instances of the test client, leading to occasional errors when
running the full test suite.

.github/workflows/spellcheck.yml

@@ -15,7 +15,7 @@ jobs:
       with:
         python-version: '3.11'
         cache: 'pip'
-        cache-dependency-path: '**/setup.py'
+        cache-dependency-path: '**/pyproject.toml'
     - name: Install dependencies
       run: |
         pip install -e '.[docs]'