[3.9] gh-95778: CVE-2020-10735: Prevent DoS by very large int() (#96502)

* Correctly pre-check for int-to-str conversion (#96537)

Converting a large enough `int` to a decimal string raises `ValueError` as expected. However, the raise comes _after_ the quadratic-time base-conversion algorithm has run to completion. For effective DOS prevention, we need some kind of check before entering the quadratic-time loop. Oops! =)

The quick fix: essentially we catch _most_ values that exceed the threshold up front. Those that slip through will still be on the small side (read: sufficiently fast), and will get caught by the existing check so that the limit remains exact.

The justification for the current check. The C code check is:
```c
max_str_digits / (3 * PyLong_SHIFT) <= (size_a - 11) / 10
```

In GitHub markdown math-speak, writing $M$ for `max_str_digits`, $L$ for `PyLong_SHIFT` and $s$ for `size_a`, that check is:
$$\left\lfloor\frac{M}{3L}\right\rfloor \le \left\lfloor\frac{s - 11}{10}\right\rfloor$$

From this it follows that
$$\frac{M}{3L} < \frac{s-1}{10}$$
hence that
$$\frac{L(s-1)}{M} > \frac{10}{3} > \log_2(10).$$
So
$$2^{L(s-1)} > 10^M.$$
But our input integer $a$ satisfies $|a| \ge 2^{L(s-1)}$, so $|a|$ is larger than $10^M$. This shows that we don't accidentally capture anything _below_ the intended limit in the check.

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

Co-authored-by: Gregory P. Smith [Google LLC] <greg@krypto.org>
Co-authored-by: Christian Heimes <christian@python.org>
Co-authored-by: Mark Dickinson <dickinsm@gmail.com>

Lib/test/test_xmlrpc.py

@@ -285,6 +285,16 @@ def test_load_extension_types(self):
         check('<bigdecimal>9876543210.0123456789</bigdecimal>',
               decimal.Decimal('9876543210.0123456789'))
 
+    def test_limit_int(self):
+        check = self.check_loads
+        maxdigits = 5000
+        with support.adjust_int_max_str_digits(maxdigits):
+            s = '1' * (maxdigits + 1)
+            with self.assertRaises(ValueError):
+                check(f'<int>{s}</int>', None)
+            with self.assertRaises(ValueError):
+                check(f'<biginteger>{s}</biginteger>', None)
+
     def test_get_host_info(self):
         # see bug #3613, this raised a TypeError
         transp = xmlrpc.client.Transport()