process: add type annotations check to CI runs

[plamut]

Closes #477.

To verify, examine the CI logs and check that the pytype session has indeed run.

PR checklist:

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

[plamut]

FWIW, the samples failure is flakiness, will re-run when this is ready to be merged.

[plamut]

@pradn @anguillanneuf Friendly ping, this would be a useful addition, thanks!