pygettext: help output for keywords prints the whole keyword dictionary

[tomasr8]

Bug report

Bug description:

The --keyword entry when printing the help via ./python Tools/i18n/pygettext.py --help shows the whole keyword dictionary:

    -k word                                                                                                                                                   
    --keyword=word                                                                                                                                            
        Keywords to look for in addition to the default set, which are:                                                                                       
        {'_': {0: 'msgid'}, 'gettext': {0: 'msgid'}, 'ngettext': {0: 'msgid', 1: 'msgid_plural'}, 'pgettext': {0: 'msgctxt', 1: 'msgid'}, 'npgettext': {0: 'ms
gctxt', 1: 'msgid', 2: 'msgid_plural'}, 'dgettext': {1: 'msgid'}, 'dngettext': {1: 'msgid', 2: 'msgid_plural'}, 'dpgettext': {1: 'msgctxt', 2: 'msgid'}, 'dnpg
ettext': {1: 'msgctxt', 2: 'msgid', 3: 'msgid_plural'}}                                                                                                       
                                                                                                                                                              
        You can have multiple -k flags on the command line.

This is because the format of DEFAULTKEYWORDS recently changed from a list to a dictionary.
We should just list the function names as we did before.

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Linked PRs

• gh-129911: pygettext: Fix the keyword entry in help output #129914