New Singleton enum for PatternMatchSingleton node (#8063)

## Summary

This PR adds a new `Singleton` enum for the `PatternMatchSingleton`
node.

Earlier the node was using the `Constant` enum but the value for this
pattern can only be either `None`, `True` or `False`. With the coming PR
to remove the `Constant`, this node required a new type to fill in.

This also has the benefit of narrowing the type down to only the
possible values for the node as evident by the removal of `unreachable`.

## Test Plan

Update the AST snapshots and run `cargo test`.

crates/ruff_python_parser/src/python.lalrpop

@@ -655,7 +655,7 @@ AddOpExpr: ast::ParenthesizedExpr = {
 
 LiteralPattern: ast::Pattern = {
     <location:@L> "None" <end_location:@R> => ast::PatternMatchSingleton {
-        value: ast::Constant::None,
+        value: ast::Singleton::None,
         range: (location..end_location).into()
     }.into(),
     <location:@L> "True" <end_location:@R> => ast::PatternMatchSingleton {