Fix CI workflow to install dependencies from pyproject.toml

- Replace `pip install -r requirements.txt` (which doesn’t exist) with:
  - `pip install -e .` to pull in sympy and numpy from pyproject.toml
  - `pip install pytest` so tests can run

Author: arcticoder

.github/workflows/pentagon-identity.yml

@@ -16,7 +16,9 @@ jobs:
       - uses: actions/checkout@v4
       - uses: actions/setup-python@v4
         with: { python-version: "3.x" }
-      - name: Install deps
-        run: pip install -r requirements.txt   # includes sympy
+      - name: Install dependencies
+        run: |
+          pip install -e .
+          pip install pytest
       - name: Run pentagon identity test
         run: pytest tests/test_biedenharn_elliott_identity.py -q