Avoid D301 autofix for u prefixed strings

[dhruvmanila]

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: #8402 (comment)

[github-actions]

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[charliermarsh]

Seems reasonable, though better I think we could be to remove the u since it's unnecessary in Python 3 IIRC.