[3.10] gh-125529: Avoid f-strings in the metagrammar

[encukou]

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.

Note that 3.9 and 3.11+ don't use f-strings in the metagrammar

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This affects our PR CI fail in the Check if generated files are up to date step, like here: https://github.com/python/cpython/actions/runs/11274132145/job/31352641619?pr=125255

I see several ways we could solve this:

• Remove this step from our CI
• Change our CI to require Python 3.11
• Merge this patch

I believe this patch is best:

• The failing check is relevant: There are a few parties that build 3.10 using a current Python -- and I expect it to become more common as 3.12+ becomes the “system Python” even on conservative platforms.
• Low chance of impactful breakage: OTOH, not that many people run pegen, and ones that do are likely equipped handle any failures.

But of course, it's entirely the RM's call. @pablogsal?

• Issue: 3.10's make regen-pegen-metaparser fails with 3.12+ #125529

[pablogsal]

This is a good think for now. I will work separately on a patch to allow fixing this by joining the tokens

[pablogsal]

Opened #125588

[encukou]

Thanks! But, this change is still needed to regen the 3.10 metagrammar with existing releases of 3.12.

[pablogsal]

Thanks! But, this change is still needed to regen the 3.10 metagrammar with existing releases of 3.12.

I know, but I want to fix the peg generator