Reinstate `opentargets` package on PyPI with deprecation message/explanation of discontinuation

[cthoyt]

It appears opentargets has been taken down from PyPI based on the discussion on #41. If this package won't be maintained anymore or the underlying API it interacts with is going away, then it would be much better to make a note of this and upload a final version to the endpoint, but as it is, this is very confusing.

If you're not interested in/able to do this (on a timely basis), I'd be happy to take care of uploading a stub package to PyPI explaining the situation

[laurendelong21]

As seen in the mentioned issue this is causing reproducibility issues for others where opentargets is a package dependency. Thanks, Charlie, for raising this. Would be great to reinstate opentargets on PyPI as removing it completely is inconvenient for others who relied on it