python.el: Respect process environment for remote shells

python.el: Respect process environment for remote shells

* lisp/progmodes/python.el
(python-shell-calculate-process-environment): Calculate
process-environment or tramp-remote-process-environment depending
whether current file is remote.
(python-shell-calculate-exec-path): Calculate exec-path or
tramp-remote-path depending whether current file is remote.
(python-shell-with-environment): New macro.
(python-shell-prompt-detect, python-shell-calculate-command)
(python-shell-make-comint, python-check): Use it.