os.urandom() now blocks on Linux

Issue #27776: The os.urandom() function does now block on Linux 3.17 and newer
until the system urandom entropy pool is initialized to increase the security.

This change is part of the PEP 524.

Lib/random.py

@@ -105,15 +105,6 @@ class Random(_random.Random):
 
         """
 
-        if a is None:
-            try:
-                # Seed with enough bytes to span the 19937 bit
-                # state space for the Mersenne Twister
-                a = int.from_bytes(_urandom(2500), 'big')
-            except NotImplementedError:
-                import time
-                a = int(time.time() * 256) # use fractional seconds
-
         if version == 1 and isinstance(a, (str, bytes)):
             x = ord(a[0]) << 7 if a else 0
             for c in a: