gh-108455: peg_generator: make the mypy config slightly stricter

[AlexWaygood]

This picks some low-hanging fruit when it comes to the mypy config, enabling various stricter settings that don't require large code changes:

• Enable --no-explicit-reexport
• Enable the truthy-bool and ignore-without-code error codes
• Explicitly note in our config file that --warn-unreachable can't yet be enabled. (It would take several code changes before we could enable it; it's deferred for now.)

• Issue: Run mypy on Tools/peg_generator #108455