[3.10] gh-125529: Avoid f-strings in the metagrammar (#125582)

Grammar actions need to be valid Python tokens and the accepted tokens need to be
listed in the actions mini-grammar).

In Python 3.12+ (PEP 701), f-strings are no longer STRING tokens, so pegen fails
to regenerate the metaparser on this Python version, as in:

   PYTHON_FOR_REGEN=python3.12 make regen-pegen-metaparser

Use `+` and plain strings rather than f-strings.

Co-authored-by: Lysandros Nikolaou <lisandrosnik@gmail.com>