test_rtutil: Implement our own nullcontext.

contextlib.nullcontext does not exist in Python 3.6.

tests/test_rtutil.py

@@ -55,11 +55,15 @@ def new_client():
         TICKET_DATA = testutil.RTClient.TICKET_DATA.copy()
     return RTClient()
 
+@contextlib.contextmanager
+def nullcontext(thing):
+    yield thing
+
 def new_cache(database=':memory:'):
     db = rtutil.RTLinkCache.setup(database)
     if db is None:
         print("NOTE: did not set up database cache at {}".format(database))
-        return contextlib.nullcontext(db)
+        return nullcontext(db)
     else:
         return contextlib.closing(db)