Update LICENSE to PEP639 standards

[thomasrockhu-codecov]

Now that PEP 639 has finally landed in full, it's best to use separate license SPDX and license-files: https://packaging.python.org/en/latest/specifications/pyproject-toml/#license.

setuptools hasn't yet implemented it, however:

pypa/setuptools#4629
pypa/setuptools#3596
pypa/setuptools#4759
pypa/setuptools#4706
pypa/setuptools#4728
So this is something to do in the future. For now, it would be a good idea to add a TODO code comment, so this action item is not lost.

see (comment)

[webknjaz]

UPD: it's been implemented and emits warnings.

Action items:

• use an SPDX identifier
• delete the trove classifier (no-op, since this project doesn't declare any)