fix missing classifiers, migrate to flit

[branchv]

Noticed the classifiers were missing. Also took the opportunity to

• Migrate to flit since it's much simpler and faster (pip recently did the same)
• Utilize [dependency-groups] so uv run pytest works on a clean checkout
• Remove the python-requires upper bound as it is generally discouraged (see https://iscinumpy.dev/post/bound-version-constraints/, uv completely ignores it)