Fix Python 3.8 SyntaxWarning: 'str' object is not callable

[andersk]

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)

Description:

Fixes these warnings from Python 3.8:

/usr/lib/python3/dist-packages/urwid/tests/test_canvas.py:141: SyntaxWarning: 'str' object is not callable; perhaps you missed a comma?
  assert result == expected, "got: %r expected: %r" (result, expected)
/usr/lib/python3/dist-packages/urwid/tests/test_canvas.py:145: SyntaxWarning: 'str' object is not callable; perhaps you missed a comma?
  assert result == expected, "got: %r expected: %r" (result, expected)
/usr/lib/python3/dist-packages/urwid/tests/test_canvas.py:149: SyntaxWarning: 'str' object is not callable; perhaps you missed a comma?
  assert result == expected, "got: %r expected: %r" (result, expected)
/usr/lib/python3/dist-packages/urwid/tests/test_canvas.py:232: SyntaxWarning: 'str' object is not callable; perhaps you missed a comma?
  assert result == expected, "got: %r expected: %r" (result, expected)

[coveralls]

Coverage decreased (-0.2%) to 76.861% when pulling d99af6c on andersk:patch-1 into 96a1be0 on urwid:master.

[coveralls]

Coverage decreased (-0.3%) to 76.763% when pulling d99af6c on andersk:patch-1 into 96a1be0 on urwid:master.

[tonycpsu]

Thanks. 👍