As discussed over the mailing list, reformat code using Black. This is a
one-off change to reformat all our codebase. Moving forward, we
shouldn't expect such blanket reformats. Rather, we expect each change
to already comply with the Black formatting style.

- Rename plugintestutils to testing.

- Don't run the tests from bst-plugins-template. This imports
  buildstream.plugintestutils so will have to be disabled to get
  through CI. This can be re nabled once bst-plugins-template has been
  patched.

Pylint can't know that pytest's fixtures are used in a file and
therefore reports false positives. Silencing all those errors

Pylint doesn't play well with pytest fixtures, we therefore need to
silence this error.

We want external plugins to be able to make use of the core testing utils.
This commit exposes the basic utilities which are currently in use in
bst-external plugins. If necessary, more utilities could be exposed in the
future.

Moves the following files from tests/testutils/ to
buildstream/plugintestingutils/:
o runcli.py
o integration.py

As part of this, this commit makes the following changes to runcli.py
and integration.py:
o runcli.py: Fix linting errors
o runcli.py: Add user facing documentation
o Integration.py: Add user facing documentation

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 gives us the machinery to automatically derive the correct version

    This is modified only to inform the linter to skip

  o Adds a .gitattributes file which informs git to substitute
    the buildstream/_version.py file, this is just to ensure that
    the versioning output would work if ever we used `git archive`
    to release a tarball.

  o Modifies setup.py and setup.cfg for versioneer

  o Modifies utils.py and _frontend/cli.py such as to avoid importing
    the derived version when running bash completion mode, we dont
    derive the version at completion time because this can result
    in running a subprocess (when running in developer install mode)
    and is an undesirable overhead.

  o Updates tests/frontend/version.py to expect changed version output

This also fixes #177 - the problem here was solved simply
by passing the project directory to `cli.run(project=project...)`