* progmodes/python.el: Enhancements to navigation commands.

* progmodes/python.el: Enhancements to navigation commands.
(python-nav-backward-sentence)
(python-nav-forward-sentence): Remove.
(python-nav-backward-statement, python-nav-forward-statement)
(python-nav-statement-start, python-nav-statement-end)
(python-nav-backward-block, python-nav-forward-block)
(python-nav-block-start, python-nav-block-end)
(python-nav-forward-sexp-function)
(python-info-current-line-comment-p)
(python-info-current-line-empty-p): New functions.
(python-indent-context): Use `python-nav-statement-start'.