Fix for #921: unifies versioning between JS and Python libraries

[sc1f]

This PR fixes our long-standing issue with Python versioning, Bumpversion, and assorted versioning quirks.

By replacing bumpversion with a task run as part of Lerna's lifecycle script, the semantics around maintaining two separate versioning configurations are removed.

Instead, the version script reads the version of package.json in the Python folder, which (at the time the lifecycle script runs) will have been updated to the new version specified in lerna publish or lerna version.

_version.py will have the version string written into it exactly as specified in package.json, and PerspectiveWidget will look for the EXACT version as specified in _version.py when it is loaded in Jupyterlab.

Thus, a distribution with mismatched versions between Python and Javascript will break the Jupyterlab widget, and a distribution that does not use lerna version or lerna publish as one of its steps will NOT have the version number reflected in _version.py. In short, Python distributions now must happen in lock-step with Javascript distributions, and with the exact same version string between the two distributions.

Changelog

• Removes bumpversion and replaces it with a script that reads version from package.json
• PerspectiveWidget now reads __version__ from _version.py
• docs task and Sphinx config now read __version__
• setup.py now reads __version__

[timkpaine]

@texodus this is ready to go!

[texodus]

Thanks for the PR! Looks good!

[texodus]

prob don't need js anymore here either due to #912