Resolve "incomplete patching of ophyd StatusBase"

Description

ophyd __init__.py imports Status objects (directly, or indirectly via importing other sub-modules). Because of this, the naive patching introduced in !100 (merged) is not sufficient. Alas, patching this kind of class on a whole code base at runtime, with __init__.py interfering is not something easy, because of the way Python works.

So, this MR bites the bullet and introduces a custom import hook, to execute specific code when ophyd.status is imported - instead of letting Python handling module creation it creates itself a module, with code from the original ophyd.status + the patch to do to fix the unneeded threads problem (which is the root of all this).

This kind of black magic is not nice, but works 😁

The patch itself is the same as previously, with same result as code that has been merged in Ophyd ; but it works differently, it uses mocked threading.Thread (to minimize changes). The best solution would be to rely on a newer Ophyd.

Closes #85 (closed)