release: bump version to 1.9.1-dev after 1.8.0 release
After 1.8.0 is released, merge it back into master so that 1.8.0 is part of the history of master. That means, $ git log --first-parent master will also traverse 1.8.0 and 1.8-rc*. Also bump the micro version to 1.9.1-dev to indicate that this is after 1.8.0 is out.
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