gh-95778: CVE-2020-10735: Prevent DoS by very large int() (#96499)

Integer to and from text conversions via CPython's bignum `int` type is not safe against denial of service attacks due to malicious input. Very large input strings with hundred thousands of digits can consume several CPU seconds.

This PR comes fresh from a pile of work done in our private PSRT security response team repo.

Signed-off-by: Christian Heimes [Red Hat] <christian@python.org>
Tons-of-polishing-up-by: Gregory P. Smith [Google] <greg@krypto.org>
Reviews via the private PSRT repo via many others (see the NEWS entry in the PR).

<!-- gh-issue-number: gh-95778 -->
* Issue: gh-95778
<!-- /gh-issue-number -->

I wrote up [a one pager for the release managers](https://docs.google.com/document/d/1KjuF_aXlzPUxTK4BMgezGJ2Pn7uevfX7g0_mvgHlL7Y/edit#). Much of that text wound up in the Issue. Backports PRs already exist. See the issue for links.

Lib/test/test_json/test_decode.py

@@ -2,6 +2,7 @@ import decimal
 from io import StringIO
 from collections import OrderedDict
 from test.test_json import PyTest, CTest
+from test import support
 
 
 class TestDecode:
@@ -95,5 +96,13 @@ class TestDecode:
         d = self.json.JSONDecoder()
         self.assertRaises(ValueError, d.raw_decode, 'a'*42, -50000)
 
+    def test_limit_int(self):
+        maxdigits = 5000
+        with support.adjust_int_max_str_digits(maxdigits):
+            self.loads('1' * maxdigits)
+            with self.assertRaises(ValueError):
+                self.loads('1' * (maxdigits + 1))
+
+
 class TestPyDecode(TestDecode, PyTest): pass
 class TestCDecode(TestDecode, CTest): pass