xgettext: Python: Change escape sequences interpretation to follow Python 3.

xgettext: Python: Change escape sequences interpretation to follow Python 3.

Reported by Serhii Tereshchenko <serg.partizan@gmail.com> at
<https://savannah.gnu.org/bugs/?65778>.

* gettext-tools/src/x-python.c (phase7_getuc): Update comments for Python 3.
Don't interpret \u sequences in raw bytes and raw strings.
(phase5_get): Recognize bytes prefixes 'b' and 'B'. For f-strings, invoke
phase7_getuc with interpret_unicode = true, not false.
* gettext-tools/tests/xgettext-python-1: Update for Python 3.
* gettext-tools/tests/xgettext-python-5: Likewise.
* NEWS: Mention the change.