ob-python.el: Assign variables in preamble instead of body

* lisp/ob-python.el (org-babel-execute:python): Add variable
assignment to preamble instead of body.
(org-babel-python-evaluate): Concatenate preamble (which now includes
variable assignment) to body before session evaluation.

Fixes
https://lists.gnu.org/archive/html/emacs-orgmode/2020-03/msg00126.html. For
non-session evaluation, ob-python adds indentation to the body inside
main(), but this was adding spurious indentation for multiline strings
passed through :var. This commit fixes the issue by moving variable
assignment out of the body and into the preamble.

lisp/ob-python.el

@@ -82,10 +82,14 @@ 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 (org-babel-variable-assignments:python params)))
+	   params))
          (result (org-babel-python-evaluate
 		  session full-body result-type result-params preamble)))
     (org-babel-reassemble-table
@@ -272,7 +276,8 @@ except Exception:
   "Evaluate BODY as Python code."
   (if session
       (org-babel-python-evaluate-session
-       session body result-type result-params)
+       session (concat preamble (and preamble "\n") body)
+       result-type result-params)
     (org-babel-python-evaluate-external-process
      body result-type result-params preamble)))