Workaround a false positive about indexing into refs

```
scripts-dev/release.py:495: error: Unsupported right operand type for in ("Callable[[], IterableList[Reference]]")  [operator]
scripts-dev/release.py:496: error: Value of type "Callable[[], IterableList[Reference]]" is not indexable  [index]
```

`refs` is an alias the the property `references`. Mypy gets confused by
the alias, see python/mypy#6700

Author: David Robertson

scripts-dev/release.py

@@ -492,8 +492,8 @@ def parse_version_from_module() -> Tuple[
 
 def find_ref(repo: git.Repo, ref_name: str) -> Optional[git.HEAD]:
     """Find the branch/ref, looking first locally then in the remote."""
-    if ref_name in repo.refs:
-        return repo.refs[ref_name]
+    if ref_name in repo.references:
+        return repo.references[ref_name]
     elif ref_name in repo.remote().refs:
         return repo.remote().refs[ref_name]
     else: