The latest MyPy release, 0.600, has "strict optional checking" enabled by default, which encounters a giant pile of issues that were not flagged before.

With --no-strict-optional passed as a command line flag, it still fails to infer the type of some values.

Some work is required to diagnose and fix the issues that 0.600 flags. Until then, the CI script will use version 0.521 (set in #321).