tests: fix for Flask-Cache and Python-2.6

Co-authored-by: Jiri Kuncar <jiri.kuncar@cern.ch>
Signed-off-by: Tibor Simko <tibor.simko@cern.ch>
Flask-RateLimiter v0.1.0

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>