Merge pull request #9786 from uranusjr/rtd-towncrier-pin

Pin towncrier under 19.9.0

Author: uranusjr

tools/requirements/docs.txt

@@ -1,4 +1,6 @@
 sphinx == 3.2.1
+# FIXME: Remove towncrier constraint after upgrading sphinxcontrib-towncrier.
+towncrier < 19.9.0
 furo
 myst_parser
 sphinx-copybutton