pySim-shell: fix compatibility problem with cmd2 >= 2.0.0 (Settable)

In cmd2 relase 2.0.0 the constructor of Settable adds a settable_object
parameter, which apparantly was optional at first, but then became
mandatory. Older versions must not have the settable_object parameter
but versions from 2.0.0 on require it. Let's add a version check so that
we stay compatible to cmd2 versions below and above 2.0.0.

See also: https://github.com/python-cmd2/cmd2

Commit 486734e85988d0d0160147b0b44a37759c833e8a
Author: Eric Lin <anselor@gmail.com>
Date:   2020-08-19 20:01:50

and

Commit 8f981f37eddcccc919329245b85fd44d5975a6a7
Author: Eric Lin <anselor@gmail.com>
Date: 2021-03-16 17:25:34

This commit is based on pySim gerrit change:
Ifce40410587c85ae932774144b9548b154ee8ad0

Change-Id: I38efe4702277ee092a5542d7d659df08cb0adeff
diff --git a/requirements.txt b/requirements.txt
index 4144a1c..bfdbd62 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -10,3 +10,4 @@
 termcolor
 colorlog
 pycryptodome
+packaging