---input---

f"""{foo
      = !s:20}"""

---tokens---
'f'           Literal.String.Affix
'"""'         Literal.String.Double
'{'           Literal.String.Interpol
'foo'         Name
'\n      '    Text.Whitespace
'= !s:'       Literal.String.Interpol
'20'          Literal.String.Double
'}'           Literal.String.Interpol
'"""'         Literal.String.Double
'\n'          Text.Whitespace
