Move Chicago Taxi Example to Python 3

Traceback (most recent call last):
  File "preprocess.py", line 259, in <module>
    main()
  File "preprocess.py", line 254, in main
    project=known_args.metric_reporting_project
  File "preprocess.py", line 155, in transform_data
    ('Analyze' >> tft_beam.AnalyzeDataset(preprocessing_fn)))
  File "/Users/kamilwasilewski/proj/beam/sdks/python/apache_beam/transforms/ptransform.py", line 987, in __ror__
    return self.transform.__ror__(pvalueish, self.label)
  File "/Users/kamilwasilewski/proj/beam/sdks/python/apache_beam/transforms/ptransform.py", line 547, in __ror__
    result = p.apply(self, pvalueish, label)
  File "/Users/kamilwasilewski/proj/beam/sdks/python/apache_beam/pipeline.py", line 532, in apply
    return self.apply(transform, pvalueish)
  File "/Users/kamilwasilewski/proj/beam/sdks/python/apache_beam/pipeline.py", line 573, in apply
    pvalueish_result = self.runner.apply(transform, pvalueish, self._options)
  File "/Users/kamilwasilewski/proj/beam/sdks/python/apache_beam/runners/runner.py", line 193, in apply
    return m(transform, input, options)
  File "/Users/kamilwasilewski/proj/beam/sdks/python/apache_beam/runners/runner.py", line 223, in apply_PTransform
    return transform.expand(input)
  File "/Users/kamilwasilewski/proj/beam/build/gradleenv/2022703441/lib/python3.7/site-packages/tensorflow_transform/beam/impl.py", line 825, in expand
    input_metadata))
  File "/Users/kamilwasilewski/proj/beam/build/gradleenv/2022703441/lib/python3.7/site-packages/tensorflow_transform/beam/impl.py", line 716, in expand
    output_signature = self._preprocessing_fn(copied_inputs)
  File "preprocess.py", line 102, in preprocessing_fn
    _fill_in_missing(inputs[key]),
KeyError: 'company'