Support Python3 syntax

[adithyagokhale]

I was migrating few modules to python3 in my project and noticed that apputils are still not compliant with python3 syntax. With these changes I was able to proceed with the migration.

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[adithyagokhale]

@googlebot I signed it!

[gpshead]

Do the things found using apputils use any APIs that would prevent them from migrating to the supported replacement for apputils and gflags known as https://pypi.org/project/absl-py/?

[gpshead]

These except transformations are incorrect. The old except , syntax should become except flags.FlagsError as error: