Add flakehell to CI run

[shardros]

It's reasonably difficult to make sure that shepherd always works from a CI perspective,we have raspi only dependencies but no raspi runner.

The pylint checks at the moment are silenced as we have so many warnings.

Adding flakehell would let us maintain at least a baseline for not introducing any more code which doesn't pass pylint and other static analysis tools.

This would let us check that our python code should in theory at least run.