gh-141271: fix f/t-str set expr quotes and nested comments

[tom-pytel]

Fix more complex cases of f/t-string quotes and nested f/t-strings. This is initial quick job so there is chance I missed something so please have a good look.

Question remaining, should the comments of NESTED f/t-strings show up in the parent f/t-string expression or not. In other words, should the following:

>>> print(f'{ # comment A
... (f'''
... # not comment B
... { # comment C '
... 3 # comment D
... * 2}''', '\n# not comment E\n6')=}')

Evaluate to:

(f'''
# not comment B
{ 
3 
* 2}''', '\n# not comment E\n6')=('\n# not comment B\n6', '\n# not comment E\n6')

Or (parent # comment A stripped, nested comments left):

(f'''
# not comment B
{ # comment C '
3 # comment D
* 2}''', '\n# not comment E\n6')=('\n# not comment B\n6', '\n# not comment E\n6')

Currently is first case, all stripped.

• Issue: certain f/t-string expressions not copied correctly to debug str or interpolations #141271

[tom-pytel]

Ping @pablogsal, @lysnikolaou