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.

When the missing variable was not defined, we would get an exception
as a string doesn't contain provenance information.

- Add a test to prevent regression

Now that both are equivalent, we can skip the sanitization part before
the yaml call.

- 'get_scalar()' allows retrieving a scalar node from a mapping.
- 'as_str()' casts a ScalarNode into a string (thus removing the node
  information).

Both together, those replace 'node_get(mapping, key, type=str)' but
also allow retrieving the 'Node' itself, which will allow in the future
lazier provenance computation.

- 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.

Replace YAML internals with a new Node type, and refactor everything
to use it cleanly.

This work was also by James Ennis <james.ennis@codethink.co.uk>

Signed-off-by: Daniel Silverstone <daniel.silverstone@codethink.co.uk>

Replace some popular copypasta.

This important-looking invocation:

    os.path.join(datafiles.dirname, datafiles.basename)

is equivalent to this shorter invocation:

    project = str(datafiles)

It seems like it's very popular copypasta, replace it with the shorter
one thus:

    # Use 'gsed' or 'sed' etc. as appropriate for your system.
    git config --global alias.sub '!f() { git grep --name-only --null "$1" | gxargs --null gsed --in-place --expression "s/$1/$2/g" ; }; f'
    git sub 'os.path.join(datafiles.dirname, datafiles.basename)' 'str(datafiles)'

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.

- Remove all wrong-import-order from pylint
- Order some subgroups of imports