Upgrade sphinxcontrib-towncrier for docs
sphinx == 3.2.1
-# FIXME: Remove towncrier constraint after upgrading sphinxcontrib-towncrier.
-towncrier < 19.9.0
+towncrier
furo
myst_parser
sphinx-copybutton
sphinx-inline-tabs
-sphinxcontrib-towncrier
+sphinxcontrib-towncrier >= 0.2.0a0
# `docs.pipext` uses pip's internals to generate documentation. So, we install
# the current directory to make it work.
