Fix RTD build: pin virtualenv<21 for hatch compatibility

Workaround for pypa/hatch#2193 — hatch is incompatible with
virtualenv 21+. Same fix used by scanpy and squidpy.

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

Author: timtreis

.readthedocs.yaml

@@ -12,5 +12,6 @@ build:
       - asdf global uv latest
     build:
       html:
-        - uvx hatch run docs:build
+        # TODO: remove "--with=virtualenv<21" once hatch is compatible with virtualenv 21+ (pypa/hatch#2193)
+        - uvx "--with=virtualenv<21" hatch run docs:build
         - mv docs/_build $READTHEDOCS_OUTPUT