pyproject.toml: use SPDX identifier to describe license

[zacchiro]

Rationale: by specifying the license as a file (like before), the full text of the license is included in the package metadata, and shown in various places, e.g., in the output of pip show, which is quite cumbersome. With this change, we use the alternative way of specifying license (namely: using a standard SPDX identifier), which makes license metadata concise and, incidentally, also machine readable.

[dnicolodi]

Thanks @zacchiro. Doing this is on my todo list. However, this assumes PEP 729 support in the build backend, thus it must come with a bump in the required setuptools version. Also, having the license specified with a classifier is deprecated when specifying the license with a SPDX identifier in the license filed, and setuptools makes it an hard metadata validation error, thus the classifier should be removed.

[dnicolodi]

The docs / build aqnd checks / build CI failures are related to this PR. The others are not. It seems that beancount git master changed something in the serialization of amounts.

[dnicolodi]

I switched to using the SPDX license identifier in #259 alongside some fixes to make the CI happy. Thanks @zacchiro.