bpo-42914: add a pprint underscore_numbers option (GH-24864)

pprint() gains a new boolean underscore_numbers kwarg to emit
integers with thousands separated by an underscore character
for improved readability (for example 1_000_000 instead of 1000000).

Lib/test/test_pprint.py

@@ -206,6 +206,7 @@ class QueryTestCase(unittest.TestCase):
             self.assertEqual(pprint.pformat(simple), native)
             self.assertEqual(pprint.pformat(simple, width=1, indent=0)
                              .replace('\n', ' '), native)
+            self.assertEqual(pprint.pformat(simple, underscore_numbers=True), native)
             self.assertEqual(pprint.saferepr(simple), native)
 
     def test_container_repr_override_called(self):
@@ -323,6 +324,18 @@ class QueryTestCase(unittest.TestCase):
      '1 '
      '2']]]]]""")
 
+    def test_integer(self):
+        self.assertEqual(pprint.pformat(1234567), '1234567')
+        self.assertEqual(pprint.pformat(1234567, underscore_numbers=True), '1_234_567')
+
+        class Temperature(int):
+            def __new__(cls, celsius_degrees):
+                return super().__new__(Temperature, celsius_degrees)
+            def __repr__(self):
+                kelvin_degrees = self + 273.15
+                return f"{kelvin_degrees}°K"
+        self.assertEqual(pprint.pformat(Temperature(1000)), '1273.15°K')
+
     def test_sorted_dict(self):
         # Starting in Python 2.5, pprint sorts dict displays by key regardless
         # of how small the dictionary may be.