On windows, test_httpservers fails with the error:

  File "C:\python\py3k\Lib\http\server.py", line 1104, in run_cgi
    exec(open(scriptfile).read(), {"__name__": "__main__"})
  File "<string>", line 3, in <module>
  File "C:\python\py3k\Lib\socket.py", line 222, in write
    return self._sock.send(b)
  TypeError: send() argument 1 must be bytes or read-only buffer, not str

There are two problems there:
- Lib/http/server.py still looks for os.popen2 or os.popen3 to start a
subprocess; these functions have been removed, this code should be
rewritten using the subprocess module.
- the fallback method, which exec() the cgi script in the python
interpreter, fails because sys.stdout should be set to a TextIOWrapper,
not a binary socket file.

Certainly the fallback method could be removed: every platform
implements subprocess nowadays.

The attached patch uses subprocess when fork is not available.
test_httpservers now passes on windows

Committed change r64389.