Comply with pep 8 style guide for backslash in assignment (Bug#24809)

* lisp/progmodes/python.el (python-indent--calculate-indentation):
Increase indent by `python-indent-offset' after
`:after-backslash-assignment-continuation'.

Copyright-paperwork-exempt: yes

lisp/progmodes/python.el

@@ -1048,13 +1048,13 @@ possibilities can be narrowed to specific indentation points."
            (max line-indentation base-indent)))
         (`(,(or :after-block-start
                 :after-backslash-first-line
+                :after-backslash-assignment-continuation
                 :inside-paren-newline-start) . ,start)
          ;; Add one indentation level.
          (goto-char start)
          (+ (current-indentation) python-indent-offset))
         (`(,(or :inside-paren
                 :after-backslash-block-continuation
-                :after-backslash-assignment-continuation
                 :after-backslash-dotted-continuation) . ,start)
          ;; Use the column given by the context.
          (goto-char start)