Bump towncrier from 19.2.0 to 21.3.0

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] <[email protected]>

Author: dependabot-preview[bot]

docs-requirements.txt

@@ -18,8 +18,12 @@ certifi==2020.12.5
     # via requests
 chardet==4.0.0
     # via requests
-click==7.1.2
+click-default-group==1.2.2
     # via towncrier
+click==7.1.2
+    # via
+    #   click-default-group
+    #   towncrier
 docutils==0.16
     # via sphinx
 idna==2.10
@@ -79,7 +83,7 @@ sphinxcontrib-trio==1.1.2
     # via -r docs-requirements.in
 toml==0.10.2
     # via towncrier
-towncrier==19.2.0
+towncrier==21.3.0
     # via -r docs-requirements.in
 urllib3==1.26.4
     # via requests