Unfortunately, the auto-import feature in PyLance surprised me with random imports.
e.g. with from unittest.case import expectedFailure just when I typed assert result == expected.
Luckily, the developers listened to the users, and with version 2020.8.0 this “feature” is optional.
In order to deactivate it, set the following option to false:
python.analysis.autoImportCompletions
Thank you, Savannah!