Remove deprecated license classifier (PEP 639)

[abravalheri]

License classifiers are deprecated post-PEP 639.
With the implementation of the PEP now this causes a warning,
so this PR removes the classifier.

Since setuptools automatically picks up LICENSE* files, the license information is already included in the distribution.

A possible additional step is to add a SPDX license expression like license = "MIT" to the [project] table in PyPI.

This PR does not implement this additional step because my understanding is that not all projects can derive accurate SPDX license expressions. For example, setuptools, as far as I understand, would need a very complex compound SPDX expression to cope with the vendored packages: MIT AND APACHE-2 AND LGPL-3.0-or-later AND PSF-2.0.... Moreover this expression would need reviewing/updating everytime the vendored dependencies are updated.

[jaraco]

In 4e3863c, I also replaced the license file with the SPDX identifier.