'"is" with a literal. Did you mean "=="?' Python warning in zip.py

This bug was originally reported by Boyuan Yang (byang@debian.org) in Debian bug #945531:

Package: diffoscope
Severity: minor
Version: 132

-> % LC_ALL=3DC sudo dpkg-reconfigure diffoscope
/usr/lib/python3/dist-packages/diffoscope/comparators/zip.py:88:
SyntaxWarning: "is" with a literal. Did you mean "=3D=3D"?
  return 0 if returncode is 3 else returncode
-> % diffoscope --version
diffoscope 132

Please consider suppressing this warning in the next release.