Show repr of the value in warnings

[ezio-melotti]

Describe the bug

A markup like :ref:`Python <test >` results in this warning: WARNING: undefined label: test .
By looking at it in the terminal it's difficult to notice the trailing space.
By using the repr of the value instead, the warning will show WARNING: undefined label: 'test '.

How to Reproduce

:ref:`Python <test >`

Expected behavior

WARNING: undefined label: 'test ' instead of WARNING: undefined label: test .

Your project

Screenshots

No response

OS

Linux

Python version

3.10

Sphinx version

Sphinx v4.5.0

Sphinx extensions

No response

Extra tools

No response

Additional context

sphinx-contrib/sphinx-lint#27 (comment)

[tk0miya]

We can use a single quote for the label. So the "repr" form is a bit confusable. I'm not sure what is better message for a label including a single quote.

This is an example using repr():

/Users/tkomiya/work/tmp/doc/index.rst:6: WARNING: undefined label: "i\'m happy"

[ezio-melotti]

See #10439 (comment).

[AA-Turner]

#10439