Clarify SyntaxWarning with literal comparison

[hauntsaninja]

I had a coworker slightly confused about the following warning, since None is a literal and is not None is idiomatic:

>>> 'a' is not None
<stdin>:1: SyntaxWarning: "is not" with a literal. Did you mean "!="?
True

I think this would be better as:

>>> 'a' is not None
<stdin>:1: SyntaxWarning: "is not" with str literal. Did you mean "!="?
True

Linked PRs

• gh-103492: Clarify SyntaxWarning with literal comparison #103493

[terryjreedy]

https://docs.python.org/3/reference/lexical_analysis.html#literals

Literals are notations for constant values of some built-in types.
In particular bytes, strings, and numbers are discussed in the following.
https://docs.python.org/3/reference/expressions.html#literals is more specific.
Python supports string and bytes literals and various numeric literals:
Evaluation of a literal yields an object of the given type (string, bytes, integer, floating point number, complex number) with the given value.

None is a constant, but not a 'literal' as defined in the sections above. This is only to say that the current message is not really ambiguous. Still, the added detail may help some.

https://docs.python.org/3/reference/expressions.html#parenthesized-forms
A tuple is a 'parenthesized form' that is either empty or has at least 1 comma. A constant tuple is not really a literal, but testing I see that one is already called a 'literal'.

The test changes look good; can't review the c code.