ob-python.el: Revert fix for multiline var

* lisp/ob-python.el (org-babel-execute:python,
org-babel-python-evaluate): Move variable assignment back to body from
preamble.
* testing/lisp/test-ob-python.el (test-ob-python/multiline-var): Mark
unit test for multiline-variable as expected to fail.

Reverts 6149b6cb6 which fixed multiline vars in ob-python, but caused
a new bug (https://lists.gnu.org/archive/html/emacs-orgmode/2020-05/msg00719.html).

lisp/ob-python.el

@@ -82,14 +82,10 @@ This function is called by `org-babel-execute-src-block'."
 	 (return-val (when (and (eq result-type 'value) (not session))
 		       (cdr (assq :return params))))
 	 (preamble (cdr (assq :preamble params)))
-	 (preamble (concat preamble (and preamble "\n")
-			   (mapconcat #'identity
-				      (org-babel-variable-assignments:python params)
-				      "\n")))
          (full-body
 	  (org-babel-expand-body:generic
 	   (concat body (if return-val (format "\nreturn %s" return-val) ""))
-	   params))
+	   params (org-babel-variable-assignments:python params)))
          (result (org-babel-python-evaluate
 		  session full-body result-type result-params preamble)))
     (org-babel-reassemble-table
@@ -276,8 +272,7 @@ except Exception:
   "Evaluate BODY as Python code."
   (if session
       (org-babel-python-evaluate-session
-       session (concat preamble (and preamble "\n") body)
-       result-type result-params)
+       session body result-type result-params)
     (org-babel-python-evaluate-external-process
      body result-type result-params preamble)))
 

testing/lisp/test-ob-python.el

@@ -174,6 +174,7 @@ _ = 'failure'
 	    (org-babel-execute-src-block)))))
 
 (ert-deftest test-ob-python/multiline-var ()
+  :expected-result :failed
   (should
    (equal "a\nb\nc"
 	  (org-test-with-temp-text "#+begin_src python :var text=\"a\\nb\\nc\"