python.el: Enhancements to process environment setup.

python.el: Enhancements to process environment setup.

* lisp/progmodes/python.el (python-shell-process-environment)
(python-shell-extra-pythonpaths, python-shell-exec-path)
(python-shell-virtualenv-root): Update docstring.  Remove :safe.
(python-shell-setup-codes): Remove :safe.
(python-shell-remote-exec-path): New defcustom.
(python-shell--add-to-path-with-priority): New macro.
(python-shell-calculate-pythonpath): Give priority to
python-shell-extra-pythonpaths.  Update docstring.
(python-shell-calculate-process-environment): Give priority to
python-shell-process-environment.  Update docstring.
(python-shell-calculate-exec-path): Give priority to
python-shell-exec-path and calculated virtualenv bin directory.
Update docstring.
(python-shell-tramp-refresh-remote-path): New function.
(python-shell-with-environment): Use it when working remotely and
do not modify tramp-remote-path.  Allow nesting.
(python-shell-calculate-command): Remove useless
python-shell-with-environment call.

* test/automated/python-tests.el (python-shell-calculate-pythonpath-1)
(python-shell-calculate-pythonpath-2)
(python-shell-calculate-process-environment-6)
(python-shell-calculate-process-environment-7)
(python-shell-calculate-process-environment-8)
(python-shell-calculate-exec-path-3)
(python-shell-calculate-exec-path-4)
(python-shell-calculate-exec-path-5)
(python-shell-calculate-exec-path-6)
(python-shell-with-environment-3): New tests.
(python-shell-calculate-process-environment-2)
(python-shell-calculate-process-environment-3)
(python-shell-calculate-process-environment-4)
(python-shell-calculate-process-environment-5)
(python-shell-calculate-exec-path-1)
(python-shell-calculate-exec-path-2)
(python-shell-with-environment-1)
(python-shell-with-environment-2): Update and simplify.