Use UTF-8 encoding when reading pyproject.toml

[domdfcoding]

This avoids a UnicodeDecodeError when reading pyproject.toml files containing unicode characters on Windows. These characters may be present in a project's metadata, such as its description or an author's name.

>>> from maturin import get_config
>>> get_config()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "lib\site-packages\maturin\__init__.py", line 39, in get_config
    pyproject_toml = toml.load(fp)
  File "lib\site-packages\toml\decoder.py", line 156, in load
    return loads(f.read(), _dict, decoder)
  File "lib\encodings\cp1252.py", line 23, in decode
    return codecs.charmap_decode(input,self.errors,decoding_table)[0]
UnicodeDecodeError: 'charmap' codec can't decode byte 0x90 in position 242: character maps to <undefined>

Furthermore, the TOML specification requires files to be encoded in UTF-8.

[messense]

Thanks