Avoid `D301` autofix for `u` prefixed strings (#8495)
This PR avoids creating the fix for `D301` if the string is prefixed
with `u` i.e., it's a unicode string. The reason being that `u` and `r`
cannot be used together as it's a syntax error.
Refer:
https://github.com/astral-sh/ruff/issues/8402#issuecomment-1788783287
