[Patch #783050 from Patrick Lynch] The emulation of forkpty() is incorrect;

[Patch #783050 from Patrick Lynch] The emulation of forkpty() is incorrect;
the master should close the slave fd.

Added a test to test_pty.py that reads from the master_fd after doing
a pty.fork(); without the fix it hangs forever instead of raising an
exception.  (<crossing fingers for the buildbots>)

Backport from trunk rev. 53146.