Use Cython master for free-threaded build

Author: oscarbenjamin

.github/workflows/buildwheel.yml

@@ -234,7 +234,10 @@ jobs:
           python -c "import sys; print(sys._is_gil_enabled())"
       - run: sudo apt-get update
       - run: sudo apt-get install libflint-dev
-      - run: pip install .
+      # Need Cython master until 3.1 is released
+      - run: pip install git+https://github.com/cython/cython.git@master
+      - run: pip install -r requirements-dev.txt
+      - run: pip install --no-build-isolation .
       - run: python -m flint.test --verbose
 
   # Run SymPy test suite against python-flint master