tests/integration was lacking a __init__.py file, meaning it was not
recognized as a python package, and thus, pylint would not check
anything inside the directory.

Adding __init__.py ensures we have correct checks here.

Some imports are fixtures, that need to be in the current namespace when
pytest runs. However, pylint does not know this.

Disabling pylint checks on those imports

This is due to pytest fixtures having to be named the same as the test
arguments.

This is a pre-requisite to enable pylint on this directory

We need to do this per file as we can't blanket disable for directories.
See upstream issue: https://github.com/PyCQA/pylint/issues/618

_scheduler/jobs: refactor, defensive send_message

See merge request BuildStream/buildstream!1373

Simplify the custom 'handle_message' / 'send_message' protocol by not
requiring a message_type. These message types share a namespace with the
base Job implementation, which could cause trouble.

Introduce a new private '_send_message' to implement the old
functionality.

Subclasses are free to pack a message type into their own messages, this
isn't necessary at present and simplifies existing subclass code.