src/python/waxeye.py: Correctly look up keys in cache.

Since Python 2.2, `dict` has supported the `key in dict` syntax for determining whether a key is present in the `dict`. Consequently, `dict.has_key` was removed in Python 3.0. This commit therefore permits waxeye to operate on Python 3.0+.

(Incidentally, I was surprised that this was the only impediment to using waxeye on Python 3.4...)