Fix gc_blob_removes_uncommitted_data

Apparently the with statement leaves a local variable pointing to the
BlobFile object, which holds a reference to the blog and prevents it
from being garbage collected.

src/ZODB/tests/testblob.py

@@ -315,7 +315,7 @@ def gc_blob_removes_uncommitted_data():
     >>> fname = blob._p_blob_uncommitted
     >>> os.path.exists(fname)
     True
-    >>> blob = None
+    >>> file = blob = None
     >>> os.path.exists(fname)
     False
     """