cache.py tuple() not hashable

[tomkooij]

I ran into some problems running checkpy because the following lines failed:

CheckPy/checkpy/caches.py

Lines 47 to 49 in 67f36f1

	key = args + tuple(values) + tuple(sys.argv)
	if key not in localCache:

It specifically failed when passing a list to a function that is wrapped by this cache.

A tuple is not guaranteed to be hashable, it fails when the tuple contains an unhashable type:

([]) in {}  # TypeError: unhashable type: 'list'

Solution: use str() to create hashable versions of any object (with __str___)

Perhaps I'll create a PR sometime :-)