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

Lib/test/test_cmd_line_script.py

@@ -633,7 +633,7 @@ class CmdLineTest(unittest.TestCase):
                 stderr.splitlines()[-3:],
                 [
                     b'    foo"""',
-                    b'         ^',
+                    b'          ^',
                     b'SyntaxError: f-string: empty expression not allowed',
                 ],
             )