[py3] Fix SyntaxWarnings

[nicfit]

/home/travis/devel/exaile/xlgui/preferences/collection.py:37:
SyntaxWarning: "is not" with a literal. Did you mean "!="?
  return [v.lower() for v in default_strip_list.split(' ') if v is not '']
/home/travis/devel/exaile/xlgui/preferences/collection.py:54:
SyntaxWarning: "is not" with a literal. Did you mean "!="?
  values = [v.lower() for v in self.widget.get_text().split(' ') if v is not '']

[genodeftest]

Nice catch, thank you!

Does this mean the second line of this code is also wrong?

except Exception:
    if field is 'tracknumber':
        temp.set_tag_raw('tracknumber', [0], notify_changed=False)

Source: https://github.com/exaile/exaile/blob/py3/plugins/daapclient/__init__.py#L569

[nicfit]

I did some grepping but did not catch that one in daapclient.. but that's the same and will SyntaxWarn if hit.

[nicfit]

@genodeftest Fixed that one too.

[genodeftest]

Thank you!

For explanation, if someone else is interested: is does identity comparison, == does value comparison. Two strings with identical letters (value) may have different identity, so the old implementation was actually wrong (but probably worked due to some backward compatibility stuff or because python just tolerates everything^^).
Example: https://www.thecoderpedia.com/blog/python-string-comparison/