ARROW-1768: [Python] Fix suppressed exception in ParquetWriter.__del__

[Licht-T]

This closes ARROW-1768.

[cpcloud]

I don't think this is the correct solution. It looks like we're defining __del__ which will be called if __init__ raises an exception. If __init__ raises an exception before defining self.is_open then you'll get an AttributeError exception in __del__ when it's called.

We really should not be opening files in constructors, since Python doesn't have the same guarantees as C++ around resource initialization and destruction. We should be using a context manager here since that provides clear semantics and guarantees when __enter__ and __exit__ will run.

For now, though, can you do this instead?

def __del__(self):
    if getattr(self, 'is_open', False):
        self.close()

[cpcloud]

LGTM