gh-90716: add _pylong.py module (#96673)

Add Python implementations of certain longobject.c functions. These use
asymptotically faster algorithms that can be used for operations on
integers with many digits. In those cases, the performance overhead of
the Python implementation is not significant since the asymptotic
behavior is what dominates runtime. Functions provided by this module
should be considered private and not part of any public API.

Co-author: Tim Peters <tim.peters@gmail.com>
Co-author: Mark Dickinson <dickinsm@gmail.com>
Co-author: Bjorn Martinsson

Tools/scripts/divmod_threshold.py

@@ -0,0 +1,56 @@
+#!/usr/bin/env python3
+#
+# Determine threshold for switching from longobject.c divmod to
+# _pylong.int_divmod().
+
+from random import randrange
+from time import perf_counter as now
+from _pylong import int_divmod as divmod_fast
+
+BITS_PER_DIGIT = 30
+
+
+def rand_digits(n):
+    top = 1 << (n * BITS_PER_DIGIT)
+    return randrange(top >> 1, top)
+
+
+def probe_den(nd):
+    den = rand_digits(nd)
+    count = 0
+    for nn in range(nd, nd + 3000):
+        num = rand_digits(nn)
+        t0 = now()
+        e1, e2 = divmod(num, den)
+        t1 = now()
+        f1, f2 = divmod_fast(num, den)
+        t2 = now()
+        s1 = t1 - t0
+        s2 = t2 - t1
+        assert e1 == f1
+        assert e2 == f2
+        if s2 < s1:
+            count += 1
+            if count >= 3:
+                print(
+                    "for",
+                    nd,
+                    "denom digits,",
+                    nn - nd,
+                    "extra num digits is enough",
+                )
+                break
+        else:
+            count = 0
+    else:
+        print("for", nd, "denom digits, no num seems big enough")
+
+
+def main():
+    for nd in range(30):
+        nd = (nd + 1) * 100
+        probe_den(nd)
+
+
+if __name__ == '__main__':
+    main()