Add requests to conda env so the docs fetch_releases tests can import

The new ultraplot/tests/test_docs_fetch_releases.py loads
docs/_scripts/fetch_releases.py via importlib, which imports `requests`
at module level. The conda env used in CI didn't include it, causing
all eight tests to fail with ModuleNotFoundError.

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

Author: cvanelteren

environment.yml

@@ -21,5 +21,6 @@ dependencies:
   - pyarrow
   - cftime
   - markdown
+  - requests
   - pip:
       - pycirclize