Use classifier for license in pyproject.toml

[sevdog]

Providing the licence file in the pyproject.toml is admitted but in that way the metadata of the package is very verbose and may break tools which are intended to parse known licence names (like MPL 2.0).

It would be better to provide the appropriate classifier : LICENSE :: OSI APPROVED :: MOZILLA PUBLIC LICENSE 2.0 (MPL 2.0).

Ref: https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license

I am very confused by the statement here: https://github.com/web-push-libs/pywebpush/blob/c99d0f7997f6796eff257083baf2a0ad77cfa518/pyproject.toml#L18-L20

Since the MPL has a canonical identifier: https://spdx.org/licenses/MPL-2.0.html