gh-138281: Remove unused topsort and bump minimal version in peg_generator

[sobolevn]

Follow up to #138281
@vstinner suggested to split this semantic change.

Description from the original PR

Please, note this change:

if sys.version_info < (3, 10):
    print("ERROR: using pegen requires at least Python 3.10!", file=sys.stderr)

Technically we did not test python3.8 for a long time and used python_version = 3.10 for mypy, so I am pretty sure that 3.8 was not supported already.

I also removed topsort which was not used.

• Issue: Run ruff on peg_generator #138281

[vstinner]

LGTM