linting: type checking failures with dulwich 0.20.36

[dtrifiro]

There have been some type checking related changes in the latest dulwich release (0.20.36) which break mypy type checking:

nox > python -m mypy
scmrepo/git/backend/dulwich/__init__.py: note: In member "fetch_refspecs" of class "DulwichBackend":
scmrepo/git/backend/dulwich/__init__.py:630:33: error: Argument
"determine_wants" to "fetch" of "GitClient" has incompatible type
"Callable[[Any], Any]"; expected
"Optional[Callable[[Dict[bytes, bytes], Optional[int]], List[bytes]]]" 
[arg-type]
                    determine_wants=determine_wants,
                                    ^
Found 1 error in 1 file (checked 26 source files)
nox > Command python -m mypy failed with exit code 1
nox > Session lint failed.