Python 3.7 compatibility and PEP479

[weinshec]

With Python 3.7 the handling of StopIteration inside generators has changed. PEP479 which was introduced in Python 3.5 is now enforced and raises a RuntimeError when StopIteration is used within generators. This currently affects 3 methods, i.e.

• Geocaching.search()
• Cache.load_trackables()
• Cache.load_logbook()

which raise a non-silent deprecation warning in Python 3.6 and a RuntimeError in Python 3.7.

Our CI currently tests Python versions 3.4 and 3.5 only. However, either we should fix this (easy, simply replace every occurance of raise StopIteration by return, see PEP479) or we should restrict our supported Python versions.