babel: cleaner initialization of org-babel-python-buffers

* lisp/babel/langs/ob-python.el (org-babel-python-buffers):
  technically this should be initialized as an alist, although in
  practice this doesn't really make any functional difference because
  whenever this variable is used it has new values properly assigned
  to it, and it is supposed to start basically empty.

lisp/babel/langs/ob-python.el

@@ -117,7 +117,7 @@ Emacs-lisp table, otherwise return the results as a string."
                                                  "'" "\"" results t))))))
      results))))
 
-(defvar org-babel-python-buffers '(:default . nil))
+(defvar org-babel-python-buffers '((:default . nil)))
 
 (defun org-babel-python-session-buffer (session)
   "Return the buffer associated with SESSION."