ci: replace pipefail-unsafe pip-show|head with importlib.metadata

The previous `pip show pydata-sphinx-theme | head -3` would SIGPIPE
under bash -o pipefail (GitHub Actions default): head closes the pipe
after 3 lines, pip show exits non-zero, the step fails despite a
successful install.

Replace with a single Python invocation against importlib.metadata
that prints just the resolved version. No pipe involved, no broken
exit codes, and the output is what we actually wanted to log.

Verified locally: exit 0, prints `pydata-sphinx-theme 0.16.1`.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: igerber

.github/workflows/docs-tests.yml

@@ -131,4 +131,7 @@ jobs:
 
       - name: Confirm pydata-sphinx-theme version
         # Surface the resolved version in the log for future debugging.
-        run: pip show pydata-sphinx-theme | head -3
+        # Avoid `pip show ... | head -3`: `head` closes the pipe after 3
+        # lines and `pip show` exits non-zero under bash -o pipefail
+        # (GitHub Actions default), failing the step despite a clean install.
+        run: python -c "import importlib.metadata as m; print('pydata-sphinx-theme', m.version('pydata-sphinx-theme'))"