bpo-38643: Raise SystemError instead of crashing when PyNumber_ToBase is called with invalid base. (GH-18863)

diff --git a/Lib/test/test_capi.py b/Lib/test/test_capi.py
index ff18a21..5a85c48 100644
--- a/Lib/test/test_capi.py
+++ b/Lib/test/test_capi.py
@@ -502,6 +502,20 @@
         # Test that subtype_dealloc decref the newly assigned __class__ only once
         self.assertEqual(new_type_refcnt, sys.getrefcount(_testcapi.HeapCTypeSubclass))
 
+    def test_pynumber_tobase(self):
+        from _testcapi import pynumber_tobase
+        self.assertEqual(pynumber_tobase(123, 2), '0b1111011')
+        self.assertEqual(pynumber_tobase(123, 8), '0o173')
+        self.assertEqual(pynumber_tobase(123, 10), '123')
+        self.assertEqual(pynumber_tobase(123, 16), '0x7b')
+        self.assertEqual(pynumber_tobase(-123, 2), '-0b1111011')
+        self.assertEqual(pynumber_tobase(-123, 8), '-0o173')
+        self.assertEqual(pynumber_tobase(-123, 10), '-123')
+        self.assertEqual(pynumber_tobase(-123, 16), '-0x7b')
+        self.assertRaises(TypeError, pynumber_tobase, 123.0, 10)
+        self.assertRaises(TypeError, pynumber_tobase, '123', 10)
+        self.assertRaises(SystemError, pynumber_tobase, 123, 0)
+
 
 class TestPendingCalls(unittest.TestCase):