Only supply popen2, popen3 when fork exists.

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

Lib/os.py

@@ -454,14 +454,16 @@ otherwise return -SIG, where SIG is the signal that killed it. """
         return spawnvpe(mode, file, args[:-1], env)
 
 
-if not _exists("popen2"):
+# 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")
             import popen2
             stdout, stdin = popen2.popen2(cmd, bufsize)
             return stdin, stdout
 
-if not _exists("popen3"):
+    if not _exists("popen3"):
         def popen3(cmd, mode="t", bufsize=-1):
             assert mode[:1] in ("b", "t")
             import popen2