Make Gitea auth token optional

For e.g. testing when running with `--dry-run`, the token may not be
necessary.

updatebot.py

@@ -166,7 +166,7 @@ Project = Union[
 
 class RepoConfig(pydantic.BaseModel):
     url: str
-    token_file: Path
+    token_file: Optional[Path] = None
     branch: str = 'master'
 
     @functools.cached_property
@@ -176,16 +176,18 @@ class RepoConfig(pydantic.BaseModel):
         return urllib.parse.urlunsplit(urlparts)
 
     @functools.cached_property
-    def auth_token(self) -> str:
+    def auth_token(self) -> Optional[str]:
+        if self.token_file:
             return self.token_file.read_text().strip()
 
     def get_git_url(self) -> str:
         session = _get_session()
+        headers = {}
+        if token := self.auth_token:
+            headers['Authorization'] = f'Bearer {token}'
         r = session.get(
             self.repo_api_url,
-            headers={
-                'Authorization': f'token {self.auth_token}',
-            },
+            headers=headers,
         )
         data = r.json()
         if ssh_url := data.get('ssh_url'):