---input---
f'x = {x:+3}'

---tokens---
'f'           Literal.String.Affix
"'"           Literal.String.Single
'x = '        Literal.String.Single
'{'           Literal.String.Interpol
'x'           Name
':'           Literal.String.Interpol
'+3'          Literal.String.Single
'}'           Literal.String.Interpol
"'"           Literal.String.Single
'\n'          Text.Whitespace