Moved version definition from _version.py to __init__.py

[radarhere]

There seems to be a consensus that __version__ should truly live in __init__.py, not _version.py.

So this PR moves the version definition, and just removes _version.py.

[hugovk]

Whilst we still need to edit it as part of the release, I'd prefer to keep it in the dedicated _version.py, it makes the release that bit easier to do.