Only supply popen2, popen3 when fork exists.

(This avoids defining non-working versions of these on the Mac.)

Lib/os.py

@@ -454,6 +454,8 @@ def spawnlpe(mode, file, *args):
         return spawnvpe(mode, file, args[:-1], env)
 
 
+# Supply popen2 etc. (for Unix)
+if _exists("fork"):
     if not _exists("popen2"):
         def popen2(cmd, mode="t", bufsize=-1):
             assert mode[:1] in ("b", "t")