PyMOTW: argparse - Command line option and argument parsing.

The argparse module was added to Python 2.7 as a replacement
for optparse. The implementation of argparse supports
features that would not have been easy to add to optparse, and
that would have required backwards-incompatible API changes, so a new
module was brought into the library instead. optparse is still
supported, but is not likely to receive new features.

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