obonet v0.2.6 release

Commits

58da37b Switch license to BSD-2-Clause-Patent
fb3e05e ci: fix travis build config validation messages
db8dd77 travis: colorize pytest output
d9ea75d upgrade travis builds
5cfc983 examples/go-obonet.ipynb: use graph.nodes syntax
482d767 style: blacken code
0cf3b4a read_obo ignore_obsolete option (#15)
cf5e4fb Travis: use API token for PyPI authorization