python.el: Generate clearer shell buffer names.

* lisp/progmodes/python.el (python-shell-get-process-name)
(python-shell-internal-get-process-name): Use `buffer-name`.
(python-shell-internal-get-or-create-process): Simplify.

* test/automated/python-tests.el (python-shell-get-process-name-1)
(python-shell-internal-get-process-name-1): Cleanup.
(python-shell-get-process-name-2)
(python-shell-internal-get-process-name-2): New tests.
(python-shell-calculate-command-1)
(python-shell-calculate-process-environment-3)
(python-shell-calculate-exec-path-2, python-shell-make-comint-1)
(python-shell-make-comint-2, python-shell-make-comint-4)
(python-shell-get-process-1, python-util-clone-local-variables-1):
Replace obsolete function and variable references with current.