D301 autofix doubles escaped backslashes

[kbattocchi]

D301 states that docstrings with literal backslashes should use raw (r""") strings. So something like

def f():
    """This docstring has a backslash \\ in it"""
    pass

should (perhaps, see below for more thoughts) instead be written as

def f():
    r"""This docstring has a backslash \ in it"""
    pass

However, the autofix for this issue just adds the r modifier to the existing string, so the fix results in

def f():
    r"""This docstring has a backslash \\ in it"""
    pass

which now has two actual backslashes. For some escape sequences this is probably the right thing to do (e.g. C:\venv in a docstring probably intends a backslash before v rather than a vertical tab before e) and so ignoring the escape sequence and just treating the backslash as if it were literal during the fix makes sense. However, I'd argue that for escaped backslashes it's less likely that the user really wanted them doubled up.

Personally, I think that it would make sense to make an exception to D301 for escaped backslashes anyway (like there is for line continuations and escaped unicode characters). Otherwise, if you need both unicode characters and backslashes in the same string, then you need to include the unicode character inline in a raw string rather than use the unicode escape because the backslash isn't allowed to be escaped. But if the rule is going to apply, having the fix deduplicate the backslashes in this case would be nice.

[charliermarsh]

That seems reasonable, though not certain what the "general rule" would be.