[3.8] [3.9] pythongh-115349: Pin theme to fix code snippets (pythonGH-115351)

Pin theme to fix code snippets
(cherry picked from commit 3fcea41)

Co-authored-by: Hugo van Kemenade <[email protected]>

Author: hugovk

Doc/requirements.txt

@@ -17,6 +17,6 @@ blurb
 
 # The theme used by the documentation is stored separately, so we need
 # to install that as well.
-python-docs-theme
+python-docs-theme==2023.3.1
 
 -c constraints.txt