Migrate from vcversioner to setuptools-scm

vcversioner has seen no update in a few years (and upstream has failed to do anything about it). In the meantime, development around setuptools-scm has been quite active.

The rest of the Python ecosystem is also growing support for setuptools-scm: for instance, check-manifest, which I'd like to introduce to our pre-commit hooks, doesn't work at all with vcversioner but supports setuptools-scm without any tweak.

The change from vcversioner to setuptools should be a pretty straightforward search and replace operation. We'll need to replace the _version module generated by vcversioner with a call to pkg_resources, as advised in https://github.com/pypa/setuptools_scm/#setuppy-usage.

Currently:

• swh-py-template
• swh.storage
• swh.scheduler
• swh.deposit
• swh.web
• swh.loader.core
• swh.loader.mercurial
• swh.loader.svn
• swh.loader.git
• swh.indexer
• swh.search
• swh.lister
• swh.model
• swh.scanner
• swh.web.client
• swh.vault
• swh.objstorage
• swh.objstorage.replayer
• swh.docs
• swh.icinga.plugins
• swh.graph
• swh.journal

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Migrated from T2105 (view on Phabricator)