Trac #34296: GH Actions: Upload wheels to PyPI

Follow-up from #33800.

Wheels built OK in 9.7.beta7 -
https://github.com/sagemath/sage/runs/7641734792, so we can now activate
the automatic upload to PyPI.

URL: https://trac.sagemath.org/34296
Reported by: mkoeppe
Ticket author(s): Matthias Koeppe
Reviewer(s): Dima Pasechnik

Author: Release Manager

.github/workflows/dist.yml

@@ -138,6 +138,8 @@ jobs:
   upload_wheels:
     needs: build_wheels
     runs-on: ubuntu-latest
+    env:
+      CAN_DEPLOY: ${{ secrets.SAGEMATH_PYPI_API_TOKEN != '' }}
     steps:
 
       - uses: actions/download-artifact@v2