Update pyproject.toml to use license expressions instead of classifiers

[nightlark]

Python is deprecating the license classifiers and they need to be removed by early 2026.

While we're at it, we might as well look at some other modernization options for the dapper-python pyproject.toml file -- use the license expression syntax supported in hatchling 1.27 or newer, or setuptools 77.0.3 or newer (https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license).

I think hatchling 1.27 supports Python 3.8 or newer, and setuptools 77.0.3 is Python 3.9 or newer -- so perhaps this is a good time to bump up our own minimum supported version.

Using hatchling would involve changing the build backend, and using hatch-vcs instead of setuptools_scm (which is used internally by hatch-vcs).