ob-python: Convert two defvars to defconsts.

* lisp/ob-python.el (org-babel-python-wrapper-method):
(org-babel-python-pp-wrapper-method): Convert from defvar to defconst.

lisp/ob-python.el

@@ -224,13 +224,13 @@ then create.  Return the initialized session."
 
 (defvar org-babel-python-eoe-indicator "'org_babel_python_eoe'"
   "A string to indicate that evaluation has completed.")
-(defvar org-babel-python-wrapper-method
+(defconst org-babel-python-wrapper-method
   "
 def main():
 %s
 
 open('%s', 'w').write( str(main()) )")
-(defvar org-babel-python-pp-wrapper-method
+(defconst org-babel-python-pp-wrapper-method
   "
 import pprint
 def main():