Update jsonstore.py (#6230)
Update jsonstore.py (#6230)

Same change as was made to DictStore here: https://github.com/kivy/kivy/commit/09ee4b5e6f91d722b535623a05ea2504cc000045#diff-76774a9a72b8018b27fd0d6f93eea2e5

I tried to .clear() my JSON store and got the RTE "dictionary changed size during iteration". Traced it down to this line (dict.keys() does not return a list in python 3, which I use). Seems to fix my problem, just hope I'm not breaking anything else... /noob
kivy/kivy,akshayaurora/kivy,matham/kivy,inclement/kivy,matham/kivy,rnixx/kivy,inclement/kivy,akshayaurora/kivy,matham/kivy,rnixx/kivy,rnixx/kivy,matham/kivy,inclement/kivy,akshayaurora/kivy,kivy/kivy,kivy/kivy