add support for gzipped paf file

[bakerwm]

Hi @mbhall88 , how about making PafFile() support for gzipped file?

I have a lot paf files, compressed in gzipped format (saving space).
How to open these files with current paf version 0.1.3?

I tried to use xopen to open the stream instead, and it works.

# file: pafpy/paffile.py

# add xopen
from xopen import xopen

# replace line-109
self._stream = xopen(self.path)

[mbhall88]

Version 0.2.0 adds support for gzip-compressed files now: https://pafpy.xyz/#compressed-input

Let me know if you have any problems with it