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/deploy-latest.yml

@@ -25,7 +25,7 @@ jobs:
       name: Configure pip caching
       with:
         path: ~/.cache/pip
-        key: ${{ runner.os }}-pip-${{ hashFiles('**/setup.py') }}
+        key: ${{ runner.os }}-pip-${{ hashFiles('**/pyproject.toml') }}
         restore-keys: |
           ${{ runner.os }}-pip-
     - name: Install Python dependencies