Make python2 explicit in shabang

compiler/2006/Python_CodeGen.jrag

examples/duck_typing/duck_typing.py

examples/simple/example_encoder.py

examples/tcp/example_tcp_client_decoder.py

examples/user_types/example_encoder.py

examples/wiki_example/example_decoder.py

lib/c/version_compare.py

lib/python/labcomm2006/LabComm.py

test/relay_gen_c.py

test/relay_gen_cs.py

test/relay_gen_java.py

test/test_encoder_decoder.py

test/test_errors.py