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]

Lib/test/test_decimal.py currently uses the deprecated optparse module for parsing command-line arguments. This task involves migrating it to use argparse.

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

[serhiy-storchaka]

There is already a help.

$ ./python -m test.test_decimal --help
Usage: test_decimal.py [--debug] [{--skip | test1 [test2 [...]]}]

Options:
  -h, --help   show this help message and exit
  -d, --debug  shows the test number and context before each test
  -s, --skip   skip over 90% of the arithmetic tests

This is an internal utility, so more fancy output is not needed.