I was thinking about how to add the end quote and found these weird cases:
  >>> "asdf'asdf'asdf"
  "asdf'asdf'asdf"
  >>> "asdf\"asdf\"asdf"
  'asdf"asdf"asdf'
  >>> "asdf\"asdf'asdf"
  'asdf"asdf\'asdf'

This means that len(s) +2 (or 3 for bytes) != len(repr(s))
e.g.

>>> s = "\"''''''"
'"\'\'\'\'\'\''
>>> s
>>> len(s)
7
>>> len(repr(s))
15

This can lead to a weird partial trailing character 
  >>> re.match(".*", "a"*48 + "'\"")
  <_sre.SRE_Match object; span=(0, 50), match='aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa\>


This means I'll need to rethink len(group0) >= 48 as the condition for truncation (as a 30 length string can be truncated by %.50R)

Maybe it makes sense to write group0 to a temp string and then check if that's truncated and extract the quote character from that
OR
PyUnicode_FromFormat('%R', group0[:50]) # avoids trailing escape character ('\') but might be longer than 50 characters