rez-python does not pass command-line flags to script

When rez-python is used as a script interpreter (i.e. #!/usr/bin/env rez-python), command-line flags (- and -- options) are processed by rez-python instead of being passed through as arguments to the script.

Start at the rez-python command-line entry point and reproduce the issue using a script invoked through a #!/usr/bin/env rez-python shebang with short and long flags. Trace how arguments are parsed, then verify that the script receives its command-line options unchanged.

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