[python3] use inspect.getfullargspec instead of inspect.getargspec The latter is now deprecated

[utils] Fixes compatiblity of QueryCache with expected dict interface QueryCache is expected to mimick the interface of the dict built-in. The current __iter__ implementation breaks this expectation by iterating over key, value pairs instead of only the keys. This changeset fixes this issue by changing the __iter__ implementation to iterate over the keys in the cache and providing an implementation of the items method with a contract identical to the dict build-in (return a copy of the key- value pairs within the dictionary as a list).

[test/statsd] add tests for cubicweb.statsd_logger

[utils] Implements __iter__ on QueryCache In order to be able to iterate over the currently cached queries (so that they may be inspected and invalidated), this changeset implementes __iter__ on QueryCache.