updated README.md to make the tutorial work

[zyxue]

I find I need to make these edits to make the tutorial work.

for the i.encode('utf-8') part, if not specified, the error would've been

Traceback (most recent call last):
  File "wsgiref/handlers.py", line 138, in run
    self.finish_response()
  File "wsgiref/handlers.py", line 184, in finish_response
    self.write(data)
  File "wsgiref/handlers.py", line 279, in write
    assert type(data) is bytes, \
AssertionError: write() argument must be a bytes instance

[benjaminp]

Thanks. I think it would make the most sense to only support Python 3. So, add python2_compatible = False where necessary.

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