Tim Jenness's comments from Slack:

Looks like the new (yesterday) mypy 0.950 is breaking pipe_base (at least). It’s not on conda-forge yet so I haven’t tried it on daf_butler but I will make one of my PRs re-run to check.

The failure:
python/lsst/pipe/base/graph/_loadHelpers.py:254: error: Argument 1 to "UUID" has incompatible type "UUID"; expected "Optional[str]"

python/lsst/pipe/base/graph/_loadHelpers.py:254: error: Subclass of "UUID" and "str" cannot exist: would have incompatible method signatures

The nodes variable is declared as Optional[Iterable[UUID]] so nodes =

Unknown macro: {UUID(n) if isinstance(n, str) else n for n in nodes}

doesn’t seem like it can ever contain str.