Optionally compress OpenSky cache files

[aliaksei135]

Adds option to compress cache files generated when the OpenSky Impala shell is queried for data.

• Compresses all cache files for any method that calls the Impala shell if compress=True passed
• When reading, checks if file is gzipped and transparently falls back to plain text file object if not
• Tests modified to test both compressed and uncompressed scenarios

Fixes #237

[xoolive]

Thank you @aliaksei135, could you please check the fixes I implemented, in particular:

• I added a context manager to properly handle open files (those were never closed);
• I removed the try/except for two reasons:
  • you assumed a BadGzip exception meant you have a text file (it can also be a corrupted gzip file);
  • for now, the compress option will be False by default, so the exception would have been raised almost every time (which is not so great if you consider exceptions as exceptional)

If it's all good with you, I can merge that.

[xoolive]

Also, the tests were not passing in the gzip branch because cache files in text format remained there :)

[aliaksei135]

Looks like late night coding is not for me... ;)

Thanks for catching those. Fixes LGTM.

Regarding the context manager not closing files, I thought this initially as well however it seems that they are closed in this kind of construct. Here is a test snippet:

def getcache():
    return open('test', 'w')

with getcache() as f:
    f.write('123')
    print(f.closed)
print(f.closed)

False
True

Happy to go with current fixes however