fix: pass --doctest-modules explicitly in CI workflow

Since --doctest-modules was removed from pyproject.toml addopts (to
avoid slow local test collection), the CI doctest step needs to pass
it explicitly.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: leotrs

.github/workflows/test-code.yml

@@ -36,4 +36,4 @@ jobs:
         pytest --color=yes
         # run doctests
         cd ../xgi/
-        pytest --color=yes
+        pytest --doctest-modules --color=yes