Thanks! Here are a few more cases I came across with the existing implementation:

>>> f"{'a\\'b'}"
  File "<stdin>", line 1
SyntaxError: missing '}' in format string expression

I believe this is valid and should produce "a'b".

>>> f"{x!s!s}"
  File "<stdin>", line 1
SyntaxError: single '}' encountered in format string

Could use a better error message.

>>> x = 3
>>> f"{x!s{y}}"
'3y}'

Not sure how this happened.