use secrets.SystemRandom as secure random nr source

secrets.SystemRandom is defined as the most secure random source
available on the given operating system.

Change-Id: I8049cd1292674b3ced82b0926569128535af6efe

pySim/esim/saip/param_source.py

@@ -17,7 +17,7 @@
 # You should have received a copy of the GNU Affero General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
-import random
+import secrets
 import re
 from osmocom.utils import b2h
 
@@ -121,7 +121,7 @@ class DecimalRangeSource(InputExpandingParamSource):
         return cls(num_digits=len(first_str), first_value=first_value, last_value=last_value)
 
 class RandomSourceMixin:
-    random_impl = random.SystemRandom()
+    random_impl = secrets.SystemRandom()
 
 class RandomDigitSource(DecimalRangeSource, RandomSourceMixin):
     """return a different sequence of random decimal digits each"""