[pre-commit.ci] auto fixes from pre-commit.com hooks

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Author: pre-commit-ci[bot]

lib/iris/_combine/__init__.py

@@ -10,9 +10,11 @@
 This is effectively a generalised "combine cubes" operation, but it is not (yet)
 publicly available.
 """
+
 import threading
 from typing import Mapping
 
+
 class CombineOptions(threading.local):
     """A container for cube combination options.