Log Message:
py-dulwich: update to 0.21.6.

0.21.6	2023-09-02

 * index: Handle different stages of conflicted paths.
   (Kevin Hendricks, Jelmer Vernooij)

 * Improve LCA finding performance. (Kevin Hendricks)

 * client: Handle Content-Type with encoding set.
   (Antoine Lambert)

 * Only import _hashlib for type checking.
   (David Hotham)

 * Update docs regarding building dulwich without c bindings (#103)
   (Adam Plaice)

 * objects: Define a stricter return type for _parse_message
   (Vincent Lorentz)

 * Raise GitProtocolError when encountering HTTP Errors in
   HTTPGitClient. (Jelmer Vernooij, #1199)