Support 3.8 f-strings =

[rocky]

See https://docs.python.org/3/whatsnew/3.8.html#f-strings-support-for-self-documenting-expressions-and-debugging

Branch 3.8-fstring-debug-attribute has some parser grammar changes and semantic action. The semantic action needs correcting.

The test from the Python test suite that tests this is test_debug_conversion() in test/test_fstring.py