Unit tests slow due to APISpec deepcopy

[jtc42]

Unit tests are really slow once the thing fixture starts being used, because the LabThing object creates APISpec data, which invokes deepcopy. When spawning a bunch of function-scopes instances for testing, this grinds the whole thing to a halt.

Unsure a nice fix, as a module-scopes Thing runs the risk of breaking proper isolated tests.

[jtc42]

Profiler stack from unit test suite

[jtc42]

Relevant lines in APISpec https://github.com/marshmallow-code/apispec/blob/dev/src/apispec/core.py#L261

[jtc42]

This turned out to be a magnum memory leak caused by a combination of LabThings and APISpec. Now fixed.