Redesign test_split_at_fraction_exhaustive tests for Python 3

ERROR: test_split_at_fraction_exhaustive (apache_beam.io.source_test_utils_test.SourceTestUtilsTest)
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Traceback (most recent call last):
File "/usr/local/google/home/valentyn/projects/beam/clean_head/beam/sdks/python/apache_beam/io/source_test_utils_test.py", line 120, in test_split_at_fraction_exhaustive
source = self._create_source(data)
File "/usr/local/google/home/valentyn/projects/beam/clean_head/beam/sdks/python/apache_beam/io/source_test_utils_test.py", line 43, in _create_source
source = LineSource(self._create_file_with_data(data))
File "/usr/local/google/home/valentyn/projects/beam/clean_head/beam/sdks/python/apache_beam/io/source_test_utils_test.py", line 35, in _create_file_with_data
f.write(line + '\n')
File "/usr/local/google/home/valentyn/projects/beam/clean_head/beam/sdks/python/target/.tox/py3/lib/python3.5/tempfile.py", line 622, in func_wrapper
return func(*args, **kwargs)
TypeError: a bytes-like object is required, not 'str'

Also similar:

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ERROR: test_file_sink_writing (apache_beam.io.filebasedsink_test.TestFileBasedSink)
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Traceback (most recent call last):
File "/usr/local/google/home/valentyn/projects/beam/clean_head/beam/sdks/python/ apache_beam/io/filebasedsink_test.py", line 121, in test_file_sink_writing
init_token, writer_results = self._common_init(sink)
File "/usr/local/google/home/valentyn/projects/beam/clean_head/beam/sdks/python/ apache_beam/io/filebasedsink_test.py", line 103, in _common_init
writer1 = sink.open_writer(init_token, '1')
File "/usr/local/google/home/valentyn/projects/beam/clean_head/beam/sdks/python/ apache_beam/options/value_provider.py", line 133, in _f
return fnc(self, *args, **kwargs)
File "/usr/local/google/home/valentyn/projects/beam/clean_head/beam/sdks/python/ apache_beam/io/filebasedsink.py", line 185, in open_writer
return FileBasedSinkWriter(self, os.path.join(init_result, uid) + suffix)
File "/usr/local/google/home/valentyn/projects/beam/clean_head/beam/sdks/python/ apache_beam/io/filebasedsink.py", line 385, in _init_
self.temp_handle = self.sink.open(temp_shard_path)
File "/usr/local/google/home/valentyn/projects/beam/clean_head/beam/sdks/python/ apache_beam/io/filebasedsink_test.py", line 82, in open
file_handle.write('[start]')
TypeError: a bytes-like object is required, not 'str'