gh-128016: Improved invalid escape sequence warning message

[umarbutler]

This PR improves the warning shown when using an invalid escape sequence by revising it from SyntaxWarning: invalid escape sequence '\W' to "\W" is an invalid escape sequence. Such sequences will not work in the future. Did you mean "\\W"? A raw string is also an option. as proposed in #128016.

The purpose is to:

1. Make clear to users that their code will soon break if they continue using backslashes.
2. Give a simple immediate fix that is guaranteed to work in all cases: add an extra backslash.
3. Note that raw strings are also an option (and often a good option, though they can sometimes have side effects, cf Hello\n World and\or fizz buzz), as suggested by @hugovk.

At present, speaking from my own experience, the current message does not communicate very clearly the urgent need to cease using invalid escapes lest you create code that will end up broken and also educate on how to quickly fix the issue.

• Issue: Make SyntaxWarning for invalid escape sequences better reflect their intended deprecation #128016

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