gh-98366: use sphinx.locale._ as gettext() in pyspecific.py (GH-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.
(cherry picked from commit d26ee8a0a552de111b8245ef0de63e3f3f535d9f)

Co-authored-by: Wei-Hsiang (Matt) Wang <mattwang44@gmail.com>