`parser` seems like it can be an `HTMLParser. There's code in the wild
that does this without obviously being wrong. As far as I can tell from
the lxml Cython source, `parser` is a `_Baseparser`. I wasn't sure if
the `iterparse` class ought to be allowed here too. I erred on the side
of caution.

It's possible for this function to return None, if the parser has
`recover=True`. I couldn't find a good way to express this without
affecting every other call to `fromstring`. (Perhaps one could make the
Parser generic over some kind of `Recoverable` indicator type... but
that seemed like overkill)

Resolves lxml#63.

This isn't strictly true: one can apparently change the default parser.
But it's a nice refinement.

so as to not burden existing users with having to check the None-ness of
`fromstring()` return values

Co-authored-by: scoder <stefan_ml@behnel.de>