Use == instead of "is" for equality testing

[naglis]

Identity check (is, is not) with certain literals (e.g. strings, numbers) is not guaranteed by the language specification to work and issue a warning starting with Python 3.8, e.g.:

examples/browse.py:326: SyntaxWarning: "is" with a literal. Did you mean "=="?

Checklist

• I've ensured that similar functionality has not already been implemented
• I've ensured that similar functionality has not earlier been proposed and declined
• I've branched off the master or python-dual-support branch
• I've merged fresh upstream into my branch recently
• I've ran tox successfully in local environment
• I've included docstrings and/or documentation and/or examples for my code (if this is a new feature)

[coveralls]

Coverage remained the same at 77.868% when pulling f52544f on naglis:fix-syntax-warning into a5f27ba on urwid:master.

[coveralls]

Coverage remained the same at 77.868% when pulling f52544f on naglis:fix-syntax-warning into a5f27ba on urwid:master.

[coveralls]

Coverage remained the same at 77.868% when pulling f52544f on naglis:fix-syntax-warning into a5f27ba on urwid:master.

[ulidtko]

LGTM