python-mode file, py-which-shell would have been nil and the command
to use would not get set correctly.  This changes things so that 1)
the temporary file has a .py extension, 2) the temporary file is put
into python-mode, and 3) the temporary file's py-which-shell is
captured in a local `shell' variable, which is used to calculate the
command to use.  Closes SF bug #545436.

(py-parse-state): Rip out the XEmacs-specific calls to
buffer-syntactic-context, which can get quite confused if there's an
open paren in column zero say, embedded in a triple quoted string.
This was always a performance hack anyway, and computers are fast
enough now that we should be able to get away with the slower, more
portable, full-parse branch.  Closes SF bug #451841.

Update the comments at the top of the file.