The subtle problem is that before the backout there was taskcluster/taskgraph/optimize/__init__.py and after the backout there wasn't, and there was taskcluster/taskgraph/optimize.py. Python happily uses a .pyc when a .py doesn't exist, so when it found the stale taskcluster/taskgraph/optimize/__init__.pyc, it used it, rather than taskcluster/taskgraph/optimize.{py,pyc}.
Bug 1574462 Comment 12 Edit History
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The subtle problem is that before the backout there was `taskcluster/taskgraph/optimize/__init__.py` and after the backout there wasn't, and there was `taskcluster/taskgraph/optimize.py` instead. Python happily uses a `.pyc` when a `.py` doesn't exist, so when it found the stale `taskcluster/taskgraph/optimize/__init__.pyc`, it used it, rather than `taskcluster/taskgraph/optimize.{py,pyc}`.