setup.py: Read package version from our __version__ constant
setup.py: Read package version from our __version__ constant

This is the same logic I'm using in nextstrain-cli.  Note that pathlib
requires Python 3.4+, which I think is an acceptable requirement.
Modular augur is already Python 3-only.

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