setup.py: Align cmd2 minimum version with requirements.txt

As pointed out in the commit-log of Change-Id
I5186f242dbc1b770e3ab8cdca7f27d2a1029fff6 we had different minimum
versions for cmd2 in requirements.txt vs setup.py.  Let's align that.

Change-Id: I71cee0ec3ed2abec68ec567beaab13c868721dad

setup.py

@@ -21,7 +21,7 @@ setup(
         "pyscard",
         "pyserial",
         "pytlv",
-        "cmd2 >= 1.5.0, < 3.0",
+        "cmd2 >= 2.6.2, < 3.0",
         "jsonpath-ng",
         "construct >= 2.10.70",
         "bidict",