mypy 0.620 signals an error on pyannotate

[Philippe F]

Using latest mypy 0.620 run on pyannotate triggers the following errors :

pyannotate_runtime/collect_types.py:375: error: The type alias is invalid in runtime context
pyannotate_runtime/collect_types.py:379: error: The type alias is invalid in runtime context
pyannotate_runtime/collect_types.py:380: error: The type alias is invalid in runtime context

See : https://travis-ci.org/dropbox/pyannotate/jobs/420859849

This is above my level of understanding of static type usage.

[Guido van Rossum]

Thanks, fixed by aa812e6