Add new pySim.ota library, implement SIM OTA crypto

This introduces a hierarchy of classes implementing

* ETS TS 102 225 (general command structure)
* 3GPP TS 31.115 (dialects for SMS-PP)

In this initial patch only the SMS "dialect" is supported,
but it is foreseen that USSD/SMSCB/HTTPS dialects can be
added at a later point.

Change-Id: I193ff4712c8503279c017b4b1324f0c3d38b9f84
diff --git a/requirements.txt b/requirements.txt
index 4f037da..4144a1c 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -9,3 +9,4 @@
 pyyaml>=5.1
 termcolor
 colorlog
+pycryptodome