Add f-string parsing for Python 3.12 (PEP 701) (#1027)

Add f-string parsing for Python 3.12 (PEP 701) (#1027)

Since Python 3.12, f-strings are tokenized and parsed like the rest
of Python's grammar, using the new tokens FSTRING_START, FSTRING_MIDDLE
and FSTRING_END.

Make the babel message extractor concatenate these three if they're
adjacent to each other. If they're not, that means there are dynamic
substitutions, so the f-string is ignored.