docs(readme): center badges, fix Python badge; add PyPI classifiers (#15)

The pypi/pyversions badge showed "missing" because it reads trove
classifiers, which the package never published. Add the classifiers for
the next release and use a static 3.10+ badge meanwhile. Badges centered
via <p align="center"> (HTML img tags - markdown is not parsed inside).

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>