GH-101291: Add low level, unstable API for pylong (GH-101685)

Co-authored-by: Petr Viktorin <encukou@gmail.com>

Lib/test/test_capi/test_long.py

@@ -0,0 +1,39 @@
+import unittest
+import sys
+
+from test.support import import_helper
+
+# Skip this test if the _testcapi module isn't available.
+_testcapi = import_helper.import_module('_testcapi')
+
+
+class LongTests(unittest.TestCase):
+
+    def test_compact(self):
+        for n in {
+            # Edge cases
+            *(2**n for n in range(66)),
+            *(-2**n for n in range(66)),
+            *(2**n - 1 for n in range(66)),
+            *(-2**n + 1 for n in range(66)),
+            # Essentially random
+            *(37**n for n in range(14)),
+            *(-37**n for n in range(14)),
+        }:
+            with self.subTest(n=n):
+                is_compact, value = _testcapi.call_long_compact_api(n)
+                if is_compact:
+                    self.assertEqual(n, value)
+
+    def test_compact_known(self):
+        # Sanity-check some implementation details (we don't guarantee
+        # that these are/aren't compact)
+        self.assertEqual(_testcapi.call_long_compact_api(-1), (True, -1))
+        self.assertEqual(_testcapi.call_long_compact_api(0), (True, 0))
+        self.assertEqual(_testcapi.call_long_compact_api(256), (True, 256))
+        self.assertEqual(_testcapi.call_long_compact_api(sys.maxsize),
+                         (False, -1))
+
+
+if __name__ == "__main__":
+    unittest.main()