Fix undefined name issues

flake8 testing of https://github.com/google/active-qa on Python 3.7.0

$ flake8 . --count --select=E901,E999,F821,F822,F823 --show-source --statistics

./px/environments/demo.py:55:25: F821 undefined name 'raw_input'
    questions_doc_ids = raw_input(
                        ^
./px/environments/docqa.py:72:20: E999 SyntaxError: invalid syntax
               async=0,
                   ^
./px/nmt/utils/nmt_utils.py:125:15: F821 undefined name 'IOException'
        raise IOException("%s: translation file not found" % trans_file)
              ^
./px/nmt/utils/logging_utils.py:29:20: F821 undefined name 'unicode'
  if isinstance(s, unicode):
                   ^
1     E999 SyntaxError: invalid syntax
3     F821 undefined name 'raw_input'
4

google / active-qa

Fix undefined name issues #1