Add ast.Constant

Issue #26146: Add a new kind of AST node: ast.Constant. It can be used by
external AST optimizers, but the compiler does not emit directly such node.

An optimizer can replace the following AST nodes with ast.Constant:

* ast.NameConstant: None, False, True
* ast.Num: int, float, complex
* ast.Str: str
* ast.Bytes: bytes
* ast.Tuple if items are constants too: tuple
* frozenset

Update code to accept ast.Constant instead of ast.Num and/or ast.Str:

* compiler
* docstrings
* ast.literal_eval()
* Tools/parser/unparse.py

Tools/parser/unparse.py

@@ -343,6 +343,11 @@ class Unparser:
         value = t.s.replace("{", "{{").replace("}", "}}")
         write(value)
 
+    def _fstring_Constant(self, t, write):
+        assert isinstance(t.value, str)
+        value = t.value.replace("{", "{{").replace("}", "}}")
+        write(value)
+
     def _fstring_FormattedValue(self, t, write):
         write("{")
         expr = io.StringIO()
@@ -364,6 +369,25 @@ class Unparser:
     def _Name(self, t):
         self.write(t.id)
 
+    def _write_constant(self, value):
+        if isinstance(value, (float, complex)):
+            self.write(repr(value).replace("inf", INFSTR))
+        else:
+            self.write(repr(value))
+
+    def _Constant(self, t):
+        value = t.value
+        if isinstance(value, tuple):
+            self.write("(")
+            if len(value) == 1:
+                self._write_constant(value[0])
+                self.write(",")
+            else:
+                interleave(lambda: self.write(", "), self._write_constant, value)
+            self.write(")")
+        else:
+            self._write_constant(t.value)
+
     def _NameConstant(self, t):
         self.write(repr(t.value))
 
@@ -443,7 +467,7 @@ class Unparser:
     def _Tuple(self, t):
         self.write("(")
         if len(t.elts) == 1:
-            (elt,) = t.elts
+            elt = t.elts[0]
             self.dispatch(elt)
             self.write(",")
         else:
@@ -490,7 +514,8 @@ class Unparser:
         # Special case: 3.__abs__() is a syntax error, so if t.value
         # is an integer literal then we need to either parenthesize
         # it or add an extra space to get 3 .__abs__().
-        if isinstance(t.value, ast.Num) and isinstance(t.value.n, int):
+        if ((isinstance(t.value, ast.Num) and isinstance(t.value.n, int))
+           or (isinstance(t.value, ast.Constant) and isinstance(t.value.value, int))):
             self.write(" ")
         self.write(".")
         self.write(t.attr)