Python 3: do not write YAML files in binary mode

YAML files are text files and Python 3 seems to enforce this.

Traceback (most recent call last):
  File "../../bin/compare-source-package-list", line 1184, in <module>
    yaml.safe_dump(output_data, output)
  File "/usr/lib/python3/dist-packages/yaml/__init__.py", line 216, in safe_dump
    return dump_all([data], stream, Dumper=SafeDumper, **kwds)
  File "/usr/lib/python3/dist-packages/yaml/__init__.py", line 188, in dump_all
    dumper.represent(data)
  File "/usr/lib/python3/dist-packages/yaml/representer.py", line 27, in represent
    self.serialize(node)
  File "/usr/lib/python3/dist-packages/yaml/serializer.py", line 54, in serialize
    self.serialize_node(node, None, None)
  File "/usr/lib/python3/dist-packages/yaml/serializer.py", line 108, in serialize_node
    self.serialize_node(value, node, key)
  File "/usr/lib/python3/dist-packages/yaml/serializer.py", line 109, in serialize_node
    self.emit(MappingEndEvent())
  File "/usr/lib/python3/dist-packages/yaml/emitter.py", line 115, in emit
    self.state()
  File "/usr/lib/python3/dist-packages/yaml/emitter.py", line 393, in expect_first_block_mapping_key
    return self.expect_block_mapping_key(first=True)
  File "/usr/lib/python3/dist-packages/yaml/emitter.py", line 403, in expect_block_mapping_key
    self.expect_node(mapping=True, simple_key=True)
  File "/usr/lib/python3/dist-packages/yaml/emitter.py", line 244, in expect_node
    self.expect_scalar()
  File "/usr/lib/python3/dist-packages/yaml/emitter.py", line 268, in expect_scalar
    self.process_scalar()
  File "/usr/lib/python3/dist-packages/yaml/emitter.py", line 533, in process_scalar
    self.write_plain(self.analysis.scalar, split)
  File "/usr/lib/python3/dist-packages/yaml/emitter.py", line 1131, in write_plain
    self.stream.write(data)
TypeError: 'str' does not support the buffer interface