always write utf-8 documents

[davemfish]

Without specifying an encoding, it was possible to write a non-utf-8 encoded document, which then would fail to read back in using fsspec.open:

This was triggered by including a string such as "m²/person"

(C:\Users\dmf\projects\invest\env-py313) λ geometamaker validate admin_boundaries.gpkg.yml
Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "C:\Users\dmf\projects\invest\env-py313\Scripts\geometamaker.exe\__main__.py", line 7, in <module>
    sys.exit(cli())
             ~~~^^
  File "C:\Users\dmf\projects\invest\env-py313\Lib\site-packages\click\core.py", line 1161, in __call__
    return self.main(*args, **kwargs)
           ~~~~~~~~~^^^^^^^^^^^^^^^^^
  File "C:\Users\dmf\projects\invest\env-py313\Lib\site-packages\click\core.py", line 1082, in main
    rv = self.invoke(ctx)
  File "C:\Users\dmf\projects\invest\env-py313\Lib\site-packages\click\core.py", line 1697, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
                           ~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
  File "C:\Users\dmf\projects\invest\env-py313\Lib\site-packages\click\core.py", line 1443, in invoke
    return ctx.invoke(self.callback, **ctx.params)
           ~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\dmf\projects\invest\env-py313\Lib\site-packages\click\core.py", line 788, in invoke
    return __callback(*args, **kwargs)
  File "C:\Users\dmf\projects\geometamaker\src\geometamaker\cli.py", line 81, in validate
    error = geometamaker.validate(filepath)
  File "C:\Users\dmf\projects\geometamaker\src\geometamaker\geometamaker.py", line 431, in validate
    yaml_string = file.read()
  File "<frozen codecs>", line 325, in decode
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xb2 in position 971: invalid start byte