Default python-shell-interpreter to python3

* lisp/progmodes/python.el (python-shell-interpreter): Default to
python3 (bug#45655).

etc/NEWS

@@ -347,6 +347,8 @@ is set to nil, this message is inhibited.
 
 ** Python mode
 
+*** 'python-shell-interpreter' now defaults to python3 on systems with python3.
+
 *** 'C-c C-r' can now be used on arbitrary regions.
 The command previously extended the start of the region to the start
 of the line, but will now actually send the marked region, as

lisp/progmodes/python.el

@@ -2027,8 +2027,12 @@ position, else returns nil."
   :group 'python
   :safe 'stringp)
 
-(defcustom python-shell-interpreter "python"
+(defcustom python-shell-interpreter
+  (cond ((executable-find "python3") "python3")
+        ((executable-find "python") "python")
+        (t "python3"))
   "Default Python interpreter for shell."
+  :version "28.1"
   :type 'string
   :group 'python)