Create unique version string for AB dev package

[haasad]

Fixes #1193

Instead of using the date for the dev version, we create a version string from the latest tag and the number of commits since the latest tag. Eg. 2.9.4dev2. Like this there should be no more problems with conflicts for multiple dev releases on the same day. Also this versioning conveys more useful information than the date. The release date itself can always be checked on https://anaconda.org/bsteubing/activity-browser-dev/files

Checklist

• Keep pull requests small so they can be easily reviewed.
• Categorize the PR by setting a good title and adding one of the labels:
  bug, feature, ui, change, documentation, breaking, ci
  as they show up in the changelog.
• Link this PR to related issues by using closing keywords.
• Request a review from another developer.

[coveralls]

coverage: 50.944%. remained the same
when pulling 8b9a056 on fix/dev-version
into 7438c37 on master.

[marc-vdm]

Thanks a lot for having a look at this! I've added @mrvisscher as reviewer, prehaps you had already seen his tag in some places, he's our new developer :)