setup.py: Formalize our Python version requirement in the metadata
setup.py: Formalize our Python version requirement in the metadata

This means the Python packaging and installation toolchain can know what
we need.  For example, the package will refuse to install on older
versions, instead of installing but not working.

blab/nextstrain-augur,nextstrain/augur,nextstrain/augur,nextstrain/augur