Add SPDX license identifier

[cdce8p]

Hatchling 1.27.0 added support for PEP 639 license expressions.
https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license-and-license-files

Also removed the Python :: 3.8 classifier. Support for it was dropped in 09f6f17.

Metadata diff

 ...
+License-Expression: MIT
 License-File: COPYING
 ...
-Classifier: License :: OSI Approved :: MIT License
 ...
-Classifier: Programming Language :: Python :: 3.8
 ...

[Julian]

Thanks!