[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).

### Requirements for tests in various cubicweb/**/test directories. ###

## shared by several test folders
cubicweb-card == 0.5.8
docutils
Twisted < 16.0.0
webtest

## cubicweb/test
Pygments
mock
#fyzz XXX pip install fails
cubicweb-file == 1.18.0
cubicweb-localperms == 0.3.2
cubicweb-tag == 1.8.3

## cubicweb/devtools/test
flake8

## cubicweb/hooks/test
psycopg2

## cubicweb/pyramid/test
pyramid >= 1.5.0
waitress >= 0.8.9
wsgicors >= 0.3
pyramid_multiauth

## cubicweb/sobject/test
cubicweb-comment == 1.12.2