Add check for redundant u prefixed string in Python 3

[federicobond]

Is your feature request related to a problem? Please describe

In Python 3, the u prefix is no longer necessary to indicate that a string is unicode, but some programmers may put it there out of habit or because they copied the code from somewhere else.

Describe the solution you'd like

Pylint should flag instances of u-prefixed strings in Python 3 with the redundant-string-prefix code.

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I can contribute this check if there is interest in it.

[Pierre-Sassoulas]

This sound reasonable; maybe redundant-u-string-prefix would be clearer ? Also, this should probably tell the user to run 2to3 because I think 2to3 removes that prefix. Maybe the reason why the check is not implemented yet in pylint ?

[federicobond]

This sound reasonable; maybe redundant-u-string-prefix would be clearer ?

I was thinking about leaving the door open to check for f-strings without interpolation and maybe other cases.

Also, this should probably tell the user to run 2to3 because I think 2to3 removes that prefix. Maybe the reason why the check is not implemented yet in pylint ?

For projects that have already been translated to Python 3, 2to3 is not very useful and I believe many would prefer to just remove the prefix manually once it's flagged by the linter.

[Pierre-Sassoulas]

Ok, it makes sense for those that did not run 2to3 in the past.

We already have a message for f-string without interpolation :) : W1309: Using an f-string that does not have any interpolated variables (f-string-without-interpolation)

[DanielNoord]

You can assign this to me. However, I would like to have #4787 first because I would include it in the visit_const function add on line 903 there.