Allow "-j0" as an argument to regrtest, to automatically select an

appropriate number of parallel workers.

Lib/test/regrtest.py

@@ -374,6 +374,13 @@ def main(tests=None, testdir=None, verbose=0, quiet=False,
             forever = True
         elif o in ('-j', '--multiprocess'):
             use_mp = int(a)
+            if use_mp <= 0:
+                try:
+                    import multiprocessing
+                    # Use all cores + extras for tests that like to sleep
+                    use_mp = 2 + multiprocessing.cpu_count()
+                except (ImportError, NotImplementedError):
+                    use_mp = 3
         elif o == '--header':
             header = True
         elif o == '--slaveargs':