CI: Fix conflict between Pillow 10.4.0 and Numpy 1.20

Should be fixed with the next Pillow release, but will also go away when
we drop Numpy 1.20, which can happen shortly.

.github/workflows/tests-pypi.yml

@@ -64,6 +64,9 @@ jobs:
             for dep in src:
               dep = dep.split(';')[0]
               out.write(dep.replace('>=', '==') + '\n')
+            # Only needed while we support numpy 1.20
+            if fname == 'requirements.txt':
+              out.write('pillow!=10.4.0\n')
         EOF
 
     - name: Install from PyPI