Fix python-send-command vs. compilation-mode

I'm guessing this is a problem born of newer Emacs.  If you attempt
compilation-forget-errors in a buffer without compilation-mode active
then you'll get an error when it tries to clrhash a nil value from a
variable that's buffer-local to compilation-mode buffers.  (Really
compilation-shell-minor-mode in our case, or something like that; please
excuse the shorthand.)