Fix issues turned up by previous python.el change

Fix issues turned up by previous python.el change

See [1] for more discussion.
* lisp/progmodes/python.el (python--prettify-symbols-alist): Define
the obsolete alias before the variable proper, so that we correctly
get the user's settings regardless of which name they set.
(python-nav-end-of-statement): Add missing backslash.
(python-shell-send-file): Call `expand-file-name' before
`file-local-name', the expansion of "~" could be different on remote
filenames.
(python-mode): Declare prettify-symbols-alist instead of checking if
it's bound.  Use the non-obsolete python-prettify-symbols-alist name,
rather than checking if the obsolete name is bound (it always is too,
but the Emacs 24 byte compiler doesn't recognize that
define-obsolete-variable-alias defines a variable).

[1]: https://lists.gnu.org/archive/html/emacs-devel/2018-02/msg00826.html