Fix flake 8 style warnings

Ran zts-report.py and test-runner.py from ./tests/test-runner/bin/
through the 2to3 (https://docs.python.org/2/library/2to3.html).
Checked the result, fixed:

• 'maxint' -> 'maxsize' that 2to3 missed.
• 'cmp=' parameter for a 'sorted()' with a 'key=' version.
• try/except wrapping of configparser import as there are still python 2.7 systems that lack a compatibility shim

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Gregor Kopka <gregor@kopka.net>
Closes #7925
Closes #7952