bpo-40612: Fix SyntaxError edge cases in traceback formatting (GH-20072)

This fixes both the traceback.py module and the C code for formatting syntax errors (in Python/pythonrun.c). They now both consistently do the following:

- Suppress caret if it points left of text
- Allow caret pointing just past end of line
- If caret points past end of line, clip to *just* past end of line

The syntax error formatting code in traceback.py was mostly rewritten; small, subtle changes were applied to the C code in pythonrun.c.

There's still a difference when the text contains embedded newlines. Neither handles these very well, and I don't think the case occurs in practice.

Automerge-Triggered-By: @gvanrossum