Bump dulwich from 0.19.16 to 0.20.6

Bumps [dulwich](https://github.com/dulwich/dulwich) from 0.19.16 to 0.20.6.
- [Release notes](https://github.com/dulwich/dulwich/releases)
- [Changelog](https://github.com/dulwich/dulwich/blob/master/NEWS)
- [Commits](jelmer/dulwich@dulwich-0.19.16...dulwich-0.20.6)

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

requirements.txt

@@ -3,7 +3,7 @@ click
 configupdater
 cssutils
 domdf-python-tools>=0.4.1
-dulwich==0.19.16
+dulwich==0.20.6
 email_validator
 importlib_resources
 jinja2>=2.11.2