converters, dumb: Fix errors since latest dulwich release (0.20.50)

ShaFile.get_type was deprecated and has been removed.

New typings have been added in dulwich that trigerred a new mypy error.

swh/loader/git/converters.py

@@ -185,6 +185,7 @@ def dulwich_commit_to_revision(obj: ShaFile) -> Revision:
 
     author_timezone = None
     committer_timezone = None
+    assert commit._chunked_text is not None  # to keep mypy happy
     for (field, value) in _parse_message(commit._chunked_text):
         if field == b"author":
             m = AUTHORSHIP_LINE_RE.match(value)

swh/loader/git/dumb.py

@@ -86,7 +86,7 @@ class GitObjectsFetcher:
         commit_objects = []
         for ref in wants:
             ref_object = self._get_git_object(ref)
-            if ref_object.get_type() == Commit.type_num:
+            if ref_object.type_num == Commit.type_num:
                 commit_objects.append(cast(Commit, ref_object))
                 self.objects[b"commit"].add(ref)
             else: