Fix two ResourceWarnings in blob_transaction.txt

src/ZODB/tests/blob_transaction.txt

@@ -155,8 +155,10 @@ connections should result in a write conflict error::
     >>> root4 = database.open(transaction_manager=tm2).root()
     >>> blob1c3 = root3['blob1']
     >>> blob1c4 = root4['blob1']
-    >>> blob1c3fh1 = blob1c3.open('a').write(b'this is from connection 3')
-    >>> blob1c4fh1 = blob1c4.open('a').write(b'this is from connection 4')
+    >>> with blob1c3.open('a') as blob1c3fh1:
+    ...     _ = blob1c3fh1.write(b'this is from connection 3')
+    >>> with blob1c4.open('a') as blob1c4fh1:
+    ...     _ = blob1c4fh1.write(b'this is from connection 4')
     >>> tm1.commit()
     >>> with root3['blob1'].open('r') as fp: fp.read()
     'this is blob 1woot!this is from connection 3'