---input---

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

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