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/test.yml

@@ -19,7 +19,7 @@ jobs:
         python-version: ${{ matrix.python-version }}
         allow-prereleases: true
         cache: pip
-        cache-dependency-path: setup.py
+        cache-dependency-path: pyproject.toml
     - name: Build extension for --load-extension test
       run: |-
         (cd tests && gcc ext.c -fPIC -shared -o ext.so)