Only install mypy if necessary
Only install mypy if necessary

Summary:
Tracking head at every commit exposes us potential flakiness and costs us ~9s per
mypy run. By using the version in the snapshot, it becomes more consistent with
other external dependencies, and means that our build is very unlikely to go red
due to a mypy change (we'll have to catch it in the snapshot).

Test Plan: Manual

Reviewers: anupc

Reviewed By: anupc

Subscribers: changesbot

Differential Revision: https://tails.corp.dropbox.com/D189619

dropbox/changes,dropbox/changes,dropbox/changes,dropbox/changes