New customization variable for python-mode indentation (Bug#28475)

* lisp/progmodes/python.el (python-indent-def-block-scale): New variable.
(python-indent--calculate-indentation): Let it control how many indent
levels are inserted for multi-line function signatures.

Copyright-paperwork-exempt: yes

etc/NEWS

@@ -1037,6 +1037,13 @@ located and whether GnuPG's option '--homedir' is used or not.
 ---
 *** Deleting a package no longer respects 'delete-by-moving-to-trash'.
 
+** Python
+
++++
+*** The new variable 'python-indent-def-block-scale' has been added.
+It controls the depth of indentation of arguments inside multi-line
+function signatures.
+
 ** Tramp
 
 +++

lisp/progmodes/python.el

@@ -752,6 +752,12 @@ It makes underscores and dots word constituent chars.")
   :type '(repeat symbol)
   :group 'python)
 
+(defcustom python-indent-def-block-scale 2
+  "Multiplier applied to indentation inside multi-line def blocks."
+  :version "26.1"
+  :type 'integer
+  :safe 'natnump)
+
 (defvar python-indent-current-level 0
   "Deprecated var available for compatibility.")
 
@@ -1071,9 +1077,9 @@ possibilities can be narrowed to specific indentation points."
                          (current-indentation)))
                      opening-block-start-points))))
         (`(,(or :inside-paren-newline-start-from-block) . ,start)
-         ;; Add two indentation levels to make the suite stand out.
          (goto-char start)
-         (+ (current-indentation) (* python-indent-offset 2))))))
+         (+ (current-indentation)
+            (* python-indent-offset python-indent-def-block-scale))))))
 
 (defun python-indent--calculate-levels (indentation)
   "Calculate levels list given INDENTATION.