Update dulwich requirement

Updates the requirements on [dulwich](https://github.com/dulwich/dulwich) to permit the latest version.
- [Release notes](https://github.com/dulwich/dulwich/releases)
- [Changelog](https://github.com/jelmer/dulwich/blob/master/NEWS)
- [Commits](jelmer/dulwich@dulwich-0.19.16...dulwich-0.21.7)

---
updated-dependencies:
- dependency-name: dulwich
  dependency-type: direct:production
...

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

requirements.txt

@@ -5,6 +5,6 @@ attrs>=20.3.0
 click>=7.1.2
 consolekit>=1.0.0
 domdf-python-tools>=0.8.3
-dulwich<=0.20.33,>=0.19.16
+dulwich>=0.19.16,<=0.21.7
 southwark>=0.7.0
 typing-extensions>=3.7.4.3