UnicodeDecodeError when reading pyproject.toml.

[domdfcoding]

build omits the "encoding" parameter when reading pyproject.toml, resulting in a UnicodeDecodeError when the file contains unicode characters unsupported by the system encoding.

The problem code is at:

build/src/build/__init__.py

Lines 143 to 144 in ca34512

	with open(spec_file) as f:
	spec = toml.load(f)

The traceback is:

Traceback (most recent call last):
  File "C:\hostedtoolcache\windows\Python\3.6.8\x64\lib\runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "C:\hostedtoolcache\windows\Python\3.6.8\x64\lib\runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "D:\a\domdf_python_tools\domdf_python_tools\.tox\build\lib\site-packages\build\__main__.py", line 214, in <module>
    main(sys.argv[1:], 'python -m build')
  File "D:\a\domdf_python_tools\domdf_python_tools\.tox\build\lib\site-packages\build\__main__.py", line 206, in main
    build_package(args.srcdir, outdir, distributions, config_settings, not args.no_isolation, args.skip_dependencies)
  File "D:\a\domdf_python_tools\domdf_python_tools\.tox\build\lib\site-packages\build\__main__.py", line 92, in build_package
    builder = ProjectBuilder(srcdir)
  File "D:\a\domdf_python_tools\domdf_python_tools\.tox\build\lib\site-packages\build\__init__.py", line 144, in __init__
    spec = toml.load(f)
  File "D:\a\domdf_python_tools\domdf_python_tools\.tox\build\lib\site-packages\toml\decoder.py", line 156, in load
    return loads(f.read(), _dict, decoder)
  File "C:\hostedtoolcache\windows\Python\3.6.8\x64\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 223: character maps to <undefined>

The TOML specification requires files to be encoded in UTF-8, and therefore the fix is as simple as:

with open(spec_file, encoding="UTF-8") as f:
    spec = toml.load(f)

See tox-dev/tox#1908 for the same issue in tox