Ensure manifest tracks package version via module's __version__ attribute

[bruno-f-cruz]

This PR splits an independent commit from #26 to allow the project.toml manifest file to track the version of the package from a private __version__ attribute.

Users can also now get the version by calling:

import harp
version = harp.__version__

[glopesdev]

Actually I realise I do have one issue with this.

The way versioning is done currently is by using git tags instead of hard-coded strings, and I'm realising I actually really like that because you don't have to worry about updating any strings until you do the release and then uploading to pip just works consistently. Same with local builds in case there are local changes setup tools assumes a patch and pre-release suffix automatically as long as you don't hard-code things.

So I guess I would rather do this the other way around and backfill the string automatically from the git release tag, rather than having the release version be set from a string we have to manually remember to update.

Is that possible?

[bruno-f-cruz]

I would assume it needs to be done at the same time as the tag is added? Maybe we close this PR for now. At the time I wanted to use this to automatically generate a Deprecated user message but I ended up dropping it anyway.