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_ast/src/comparable.rs

@@ -185,8 +185,8 @@ pub struct PatternMatchValue<'a> {
 }
 
 #[derive(Debug, PartialEq, Eq, Hash)]
-pub struct PatternMatchSingleton<'a> {
-    value: ComparableConstant<'a>,
+pub struct PatternMatchSingleton {
+    value: ComparableSingleton,
 }
 
 #[derive(Debug, PartialEq, Eq, Hash)]
@@ -227,7 +227,7 @@ pub struct PatternMatchOr<'a> {
 #[derive(Debug, PartialEq, Eq, Hash)]
 pub enum ComparablePattern<'a> {
     MatchValue(PatternMatchValue<'a>),
-    MatchSingleton(PatternMatchSingleton<'a>),
+    MatchSingleton(PatternMatchSingleton),
     MatchSequence(PatternMatchSequence<'a>),
     MatchMapping(PatternMatchMapping<'a>),
     MatchClass(PatternMatchClass<'a>),
@@ -326,6 +326,23 @@ impl<'a> From<&'a ast::Decorator> for ComparableDecorator<'a> {
     }
 }
 
+#[derive(Debug, PartialEq, Eq, Hash)]
+pub enum ComparableSingleton {
+    None,
+    True,
+    False,
+}
+
+impl From<&ast::Singleton> for ComparableSingleton {
+    fn from(singleton: &ast::Singleton) -> Self {
+        match singleton {
+            ast::Singleton::None => Self::None,
+            ast::Singleton::True => Self::True,
+            ast::Singleton::False => Self::False,
+        }
+    }
+}
+
 #[derive(Debug, PartialEq, Eq, Hash)]
 pub enum ComparableConstant<'a> {
     None,