ctl: port shell command for python 3 (#39317)

Command was initially copied/adapted from Django, this is an almost straight
cherrypick of later commit 723c9a8c6db60108f584972498fa6bbd3b408444.

wcs/ctl/shell.py

@@ -69,16 +69,16 @@ class CmdShell(Command):
                 readline.parse_and_bind("tab:complete")
 
             # We want to honor both $PYTHONSTARTUP and .pythonrc.py, so follow system
-            # conventions and get $PYTHONSTARTUP first then import user.
+            # conventions and get $PYTHONSTARTUP first then .pythonrc.py.
             if not sub_options.plain:
-                pythonrc = os.environ.get("PYTHONSTARTUP")
-                if pythonrc and os.path.isfile(pythonrc):
-                    try:
-                        execfile(pythonrc)
-                    except NameError:
-                        pass
-                # This will import .pythonrc.py as a side-effect
-                import user
+                for pythonrc in (os.environ.get("PYTHONSTARTUP"),
+                                 os.path.expanduser('~/.pythonrc.py')):
+                    if pythonrc and os.path.isfile(pythonrc):
+                        try:
+                            with open(pythonrc) as handle:
+                                exec(compile(handle.read(), pythonrc, 'exec'))
+                        except NameError:
+                            pass
             code.interact(local=imported_objects)
 
     shells = [ 'ipython', 'bpython' ]