Fix pretty_compile command

[j1o1h1n]

I followed the tutorial from the github wiki page. There was a problem when I got to pretty_compile.

$ serpent pretty_compile mul2.se
Traceback (most recent call last):
  File "/usr/local/var/pyenv/versions/ethereum/bin/serpent", line 9, in <module>
    load_entry_point('ethereum-serpent==2.0.2', 'console_scripts', 'serpent')()
...
    print(binascii.b2a_hex(o).decode('ascii'))
TypeError: a bytes-like object is required, not 'map'

This has a trivial fix, but I couldn't run the test suite.

[beaugunderson]

i think this can be closed without merging; unable to reproduce against HEAD