Using setuptools_scm had a couple of bad problems:

  o Unexpected versioning semantics, setuptools_scm would
    increment the micro version by itself in the case that
    we derive a version number from something that is not a tag,
    making the assumption that we are "leading up to" the next
    micro version.

    People mostly dont expect this.

  o When installing in developer mode, i.e. with `pip3 install --user -e .`,
    then we were always picking the generated version at install time
    and never again dynamically resolving it.

    Many of our users install this way and update through git, so it's
    important that we report more precise versions all the time.

This commit needs to make a series of changes at the same time:

  o Adds versioneer.py to the toplevel, this is used by setup.py
    for various activities.

    This is modified only to inform the linter to skip

  o Adds buildstream/_version.py, which is generated by versioneer
    and give...