Don't use Cython beta

This was for Cython 3.0 beta to test with Python 3.13,
but it's now breaking our tests with Cython 3.1, while
Cython 3.0 stable is out.

Author: fantix

.github/workflows/tests.yml

@@ -57,8 +57,6 @@ jobs:
 
     - name: Install Python Deps
       if: steps.release.outputs.version == 0
-      env:
-        PIP_PRE: ${{ matrix.python-version == '3.13' && '1' || '0' }}
       run: |
         pip install -e .[test,dev]