gh-98366: use sphinx.locale._ as gettext() in pyspecific.py (#98437)

fix(doc-tools): use sphinx.locale._ as gettext() for backward-compatibility in pyspecific.py

[why] spinix 5.3 changed locale.translators from a defaultdict(gettext.NullTranslations) to a dict, which leads to failure of pyspecific.py. Use sphinx.locale._ as gettext to fix the issue.