Improve Lib/test/test_decimal.py CLI with argparse

[tonghuaroot]

Description

This change improves the command-line interface experience, including support for colorized help output and better grouping of arguments.

Verification

• Run python Lib/test/test_decimal.py --help
• Run python Lib/test/test_decimal.py --skip

Has this already been discussed elsewhere?

No response given

Links to previous discussion of this feature:

No response

Linked PRs

• gh-143984: Improve Lib/test/test_decimal.py CLI with argparse #143983

[tanloong]

The optparse module was actually undeprecated (see #126180) after some discussion about its continued value.