Add assertions that we are running under Python 3.

Signed-off-by: Daira Hopwood <daira@jacaranda.org>

sapling_generators.py

@@ -1,4 +1,6 @@
 #!/usr/bin/env python3
+import sys; assert sys.version_info[0] >= 3, "Python 3 required."
+
 from pyblake2 import blake2s
 
 from sapling_jubjub import Point, JUBJUB_COFACTOR

sapling_jubjub.py

@@ -1,4 +1,6 @@
 #!/usr/bin/env python3
+import sys; assert sys.version_info[0] >= 3, "Python 3 required."
+
 from sapling_utils import i2lebsp, leos2ip, i2leosp
 
 q_j = 52435875175126190479447740508185965837690552500527637822603658699938581184513

sapling_key_components.py

@@ -1,4 +1,6 @@
 #!/usr/bin/env python3
+import sys; assert sys.version_info[0] >= 3, "Python 3 required."
+
 from pyblake2 import blake2b, blake2s
 
 from sapling_generators import PROVING_KEY_BASE, SPENDING_KEY_BASE, group_hash

sapling_merkle_tree.py

@@ -1,4 +1,6 @@
 #!/usr/bin/env python3
+import sys; assert sys.version_info[0] >= 3, "Python 3 required."
+
 from binascii import unhexlify
 
 from sapling_pedersen import pedersen_hash

sapling_notes.py

@@ -1,4 +1,6 @@
 #!/usr/bin/env python3
+import sys; assert sys.version_info[0] >= 3, "Python 3 required."
+
 from pyblake2 import blake2s
 
 from sapling_pedersen import (

sapling_pedersen.py

@@ -1,4 +1,6 @@
 #!/usr/bin/env python3
+import sys; assert sys.version_info[0] >= 3, "Python 3 required."
+
 from sapling_generators import (
     find_group_hash,
     NOTE_POSITION_BASE,

sapling_signatures.py

@@ -1,4 +1,6 @@
 #!/usr/bin/env python3
+import sys; assert sys.version_info[0] >= 3, "Python 3 required."
+
 import os
 from pyblake2 import blake2b
 

sapling_utils.py

@@ -1,4 +1,5 @@
 #!/usr/bin/env python3
+import sys; assert sys.version_info[0] >= 3, "Python 3 required."
 
 def cldiv(n, divisor):
     return (n + (divisor - 1)) // divisor

tv_output.py

@@ -1,3 +1,6 @@
+#!/usr/bin/env python3
+import sys; assert sys.version_info[0] >= 3, "Python 3 required."
+
 import argparse
 from binascii import hexlify
 import json