Add autofix for RUF039

[AlexWaygood]

The newly added preview rule RUF039 doesn't have an autofix. It would be very convenient if it had one, since it's otherwise quite a noisy rule.

[MichaReiser]

At least for the common cases. Conversions to raw strings can be tricky if backslashes are involved

[MichaReiser]

Or a fix involving backslashes at least needs to be unsafe because

>>> re.compile("\n\r\t").pattern == re.compile(r"\n\r\t").pattern
False
>>> re.compile("\n\r\t").pattern
'\n\r\t'
>>> re.compile(r"\n\r\t").pattern
'\\n\\r\\t'

[AlexWaygood]

Yeah. But even an autofix just for strings that don't have any backslashes in them would make things much more ergonomic. E.g. none of the strings that I had to manually edit in python-trio/trio#3143 had any backslashes in them.

[dylwil3]

@AlexWaygood feel free to reopen if you'd like to encourage a second pass on this with more extensive auto-fixing!

[AlexWaygood]

Thanks! 😃 I think this is going to take care of most of the noise as-is, so I'm happy to leave it there for now unless we get requests from users for fixes in more cases 👍