(py-execute-file): Better interaction with comint.  Set
comint-scroll-to-bottom-on-output to t.  Wrapper buffer change in
unwind-protect in case process filter fails.

(py-shell): Start Python with -i flag to fix tty problem on Windows;
presumably -- not yet tested.

(py-clear-queue): New function to clear the pending exec file queue.
Not currently keybound.

(py-execute-region, py-execute-buffer): Added optional async flag (use
via C-u prefix) to execute the region in a new asynchrous buffer, even
if the Python shell is running.

(py-append-to-process-buffer): Removed as obsolete.  Comint provides
this functionality.

Removed fbound test defun of match-string.  All modern X/Emacsen have
this function.