python.el: Handle file encoding for shell.

* lisp/progmodes/python.el (python-rx-constituents): Add coding-cookie.
(python-shell--save-temp-file): Write file with proper encoding.
(python-shell-buffer-substring): Add coding cookie for detected
encoding to generated content.  Fix blank lines when removing
if-name-main block.
(python-shell-send-file): Handle file encoding.
(python-info-encoding-from-cookie)
(python-info-encoding): New functions.

* test/automated/python-tests.el (python-shell-buffer-substring-1)
(python-shell-buffer-substring-2, python-shell-buffer-substring-3)
(python-shell-buffer-substring-4, python-shell-buffer-substring-5)
(python-shell-buffer-substring-6, python-shell-buffer-substring-7)
(python-shell-buffer-substring-8)
(python-info-encoding-from-cookie-1)
(python-info-encoding-from-cookie-2)
(python-info-encoding-from-cookie-3)
(python-info-encoding-from-cookie-4)
(python-info-encoding-from-cookie-5)
(python-info-encoding-from-cookie-6)
(python-info-encoding-from-cookie-7, python-info-encoding-1)
(python-info-encoding-2): New tests.