param_source: use random.SystemRandom as random nr source

Python's random module uses a PRNG (Mersenne Twister) which is
utterly insecure for key generation - it was so far only used for
testing.  Replace it with random.SystemRandom(), which draws from
/dev/urandom and is suitable for generating cryptographic key material.

Change-Id: I6de38c14ac6dd55bc84d53974192509c18d02bfa
Jenkins: skip-card-test

pySim/esim/saip/param_source.py

@@ -124,7 +124,7 @@ class DecimalRangeSource(InputExpandingParamSource):
         return "%0*d" % (self.num_digits, val)  # pylint: disable=consider-using-f-string
 
 class RandomSourceMixin:
-    random_impl = random # TODO secure random source?
+    random_impl = random.SystemRandom()
 
 class RandomDigitSource(DecimalRangeSource, RandomSourceMixin):
     """return a different sequence of random decimal digits each"""