Bump towncrier from 19.2.0 to 21.3.0 (#9)

* 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)

---
updated-dependencies:
- dependency-name: towncrier
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

* 📰 Automatic changes ⚙ Adding news file

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Monty Bot <[email protected]>

Author: dependabot[bot]

news/202108110250.bugfix

@@ -0,0 +1 @@
+Dependency upgrade: towncrier-21.3.0

setup.py

@@ -53,7 +53,7 @@
     include_package_data=True,
     install_requires=[
         "gitpython",
-        "towncrier==19.2.0",
+        "towncrier==21.3.0",
         "pyautoversion~=1.2.0",
         # FIXME change when https://github.com/pdoc3/pdoc/issues/299 is fixed
         "pdoc3==0.9.1",