mrhlpr.mr: Use pass instead of ellipsis

mypyc doesn't understand the ellipsis in this context:

    mrhlpr/mr.py:177: error: Unsupported statement in class body

And they've functionally the same anyway, so just work around it like
this.

mrhlpr/mr.py

@@ -174,7 +174,8 @@ def get_artifacts_zip(mr_id: int, no_cache: bool, origin: gitlab.GitLabOrigin) -
     )
 
 
-class UnknownReleaseError(ValueError): ...
+class UnknownReleaseError(ValueError):
+    pass
 
 
 def _target_branch_to_pmos_release(target_branch: str) -> str: