Bump towncrier from 19.2.0 to 21.3.0 (#572)

Bumps [towncrier](https://github.com/hawkowl/towncrier) from 19.2.0 to 21.3.0.
- [Release notes](https://github.com/hawkowl/towncrier/releases)
- [Changelog](https://github.com/twisted/towncrier/blob/master/NEWS.rst)
- [Commits](twisted/towncrier@19.2.0...21.3.0)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

requirements/dev.txt

@@ -2,4 +2,4 @@
 -r ci.txt
 -r doc.txt
 pytest-sugar==0.9.4
-towncrier==19.2.0
+towncrier==21.3.0