"Invalid type" errors appearing on Travis but not locally

[Strilanc]

I thought I fixed these by moving the line assigning the type variable into its own file, but the error has reappeared: https://travis-ci.com/quantumlib/Cirq/jobs/126214384

@maffoo , any ideas how to solve this? I don't think it's exactly the same as the python/mypy#5015 issue we previously reported, but it looks pretty similar.

[maffoo]

I think the first step to solving this is to pin the mypy version in the requirements file to ensure that we are using the same version of the type checker everywhere. mypy is still in flux enough that these kinds of issues can come and go.

[Strilanc]

The test scripts I run locally recreate the virtualenv each time, so it should always be the same version.

But pinning the version is a good idea. We don't all PRs to start failing at the same time every time mypy is updated. Pinning to v0.590 fixes the issue.