Compatibility with pyinstaller (#2537)

it doesn't like relative imports and not recognise `http.server` imported with `six`.
The last one is explicitly added to the list of `hiddenimports()` and will break compatibility with python 2.7, which support will be dropped in the next Patroni release anyway.

Close https://github.com/zalando/patroni/issues/2535

patroni.spec

@@ -8,7 +8,7 @@ def hiddenimports():
     sys.path.insert(0, '.')
     try:
         import patroni.dcs
-        return patroni.dcs.dcs_modules()
+        return patroni.dcs.dcs_modules() + ['http.server']
     finally:
         sys.path.pop(0)