This simplifies running flake8 tests and reduces the amount of F821 errors
reported (flake8-pyi enables support for forward references in *.pyi files).
The error code is left disabled until I clean up the remaining issues.
This is currently disabled due to the high number of errors that need fixing.
I'll be committing fixes for them and updating the numbers in .flake8 until we
can enable linting for all files by default.