Add MIT license expression in pyproject.toml

[TheJulianJES]

This adds license = "MIT" to pyproject.toml. This is a valid SPDX license expression for the MIT license.

setup.py defined the license expression as well as the (now deprecated) license classifiers before d7ed47f (in 1.3.0):

wsproto/setup.py

Lines 35 to 39 in 8b0bac4

	license='MIT License',
	classifiers=[
	'Development Status :: 5 - Production/Stable',
	'Intended Audience :: Developers',
	'License :: OSI Approved :: MIT License',

(We use this at Home Assistant to make sure all dependencies have OSI approved licenses.)

[Kriechi]

Thanks! Quite a complicated topic for such a small detail!
For reference: PEP 639 and pypa/setuptools#4903 go into more detail.