Create ~/.pypirc securely (#13512).

There was a window between the write and the chmod where the user’s
password would be exposed, depending on default permissions.  Philip
Jenvey’s patch fixes it.

config.py

@@ -42,7 +42,7 @@ class PyPIRCCommand(Command):
     def _store_pypirc(self, username, password):
         """Creates a default .pypirc file."""
         rc = self._get_rc_file()
-        f = open(rc, 'w')
+        f = os.fdopen(os.open(rc, os.O_CREAT | os.O_WRONLY, 0600), 'w')
         try:
             f.write(DEFAULT_PYPIRC % (username, password))
         finally: