Fix dev version
The dev version that ends in `.dev0` should be greater than the current version.
However it happens that `1.8.0 > 1.8.0.dev0` for example.
Therefore we need to use `1.8.1.dev0` for example in this case.

I updated the dev version to use `1.8.1.dev0`, and I also added a comment in the setup.py in the release steps about this.