Specify attrs minimum required version

[albertvaka]

We use the argument kw_only in attr.s which was added in 18.2.0 [1]

[1] https://github.com/python-attrs/attrs/blob/main/CHANGELOG.rst#1820-2018-09-01

[albertvaka]

Using an older version results in:

Traceback (most recent call last):
  File "/Users/albertvaka/example.py", line 1, in <module>
    import pytiled_parser
  File "/usr/local/lib/python3.9/site-packages/pytiled_parser/__init__.py", line 15, in <module>
    from .layer import ImageLayer, Layer, LayerGroup, ObjectLayer, TileLayer
  File "/usr/local/lib/python3.9/site-packages/pytiled_parser/layer.py", line 23, in <module>
    from . import tiled_object
  File "/usr/local/lib/python3.9/site-packages/pytiled_parser/tiled_object.py", line 13, in <module>
    @attr.s(auto_attribs=True, kw_only=True)
TypeError: attrs() got an unexpected keyword argument 'kw_only'

[Cleptomania]

Thanks for the fix, will be included in a 1.6.0 release that's probably coming up sometime next week or the week after.

[albertvaka]

There's no rush whatsoever, thanks for your time :D