[conf] Pin pydata-theme&sphinx-book-theme for "Toggle primary sidebar" issue

See also executablebooks/sphinx-book-theme#865

Author: SilverRainZ

Diff for: requirements.txt

@@ -17,7 +17,12 @@ sphinxcontrib-plantuml
 sphinxcontrib.asciinema
 sphinx-reredirects
 sphinxnotes-recentupdate
-sphinx-book-theme>=1.0.0
+
+# Pinned for "Toggle primary sidebar" issue.
+# https://github.com/plone/documentation/issues/1773
+sphinx-book-theme==1.1.3
+pydata-sphinx-theme==0.15.4 
+
 sphinxnotes-mock
 sphinxext-opengraph
 sphinxnotes-extweb