KATParser: don't leak self.fp

As noted by the python 3 interpreter, self.fp is leaked every time KATParser
is called (it raises a ResourceWarning).

Refactor KATParser in order to not leak self.fp:

• Don't allocate self.fp in __init__(). Doing so requires adding self.fp.close(), and as noted before, makes for very fragile code when the interpreter is tearing down objects (it can result in crashes in some interpreter versions if triggered with the right conditions). Limit self.fp allocation/use to context suites ("the with statement") instead, as this is more likely to not cause non-deterministic weirdness issues with the interpreter.
• Rename __iter__ to __next__ as it was providing __next__ support and return the KATParser object instead with __iter__, as __iter__ is meant to return an iterator and __next__ is meant to help iterate over an iterator.