Set PYTHONUNBUFFERED on shell startup.

Fixes: debbugs:18595

* lisp/progmodes/python.el (python-shell-unbuffered): New var.
(python-shell-calculate-process-environment): Use it.

* test/automated/python-tests.el
(python-shell-calculate-process-environment-4)
(python-shell-calculate-process-environment-5): New tests.
(python-shell-make-comint-3): Use file-equal-p.
(python-shell-get-or-create-process-1)
(python-shell-get-or-create-process-2)
(python-shell-get-or-create-process-3): Fix interpreter for
Windows.