build(deps): bump twisted in /testing/plugins_integration

Bumps [twisted](https://github.com/twisted/twisted) from 20.3.0 to 21.2.0.
- [Release notes](https://github.com/twisted/twisted/releases)
- [Changelog](https://github.com/twisted/twisted/blob/twisted-21.2.0/NEWS.rst)
- [Commits](twisted/twisted@twisted-20.3.0...twisted-21.2.0)

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

Author: dependabot[bot]

testing/plugins_integration/requirements.txt

@@ -11,5 +11,5 @@ pytest-rerunfailures==9.1.1
 pytest-sugar==0.9.4
 pytest-trio==0.7.0
 pytest-twisted==1.13.2
-twisted==20.3.0
+twisted==21.2.0
 pytest-xvfb==2.0.0