Install cython in sdist build job

Signed-off-by: Jono Yang <jyang@nexb.com>

Author: JonoYang

.github/workflows/test-and-build.yml

@@ -96,7 +96,7 @@ jobs:
 
           - name: Checkout and install reqs
             run: |
-                pip install --upgrade --user build twine pkginfo packaging pip setuptools
+                pip install --upgrade --user build twine pkginfo packaging pip setuptools cython
 
           - name: Build sdist
             run: |