Install future to run mypy on Python 3

LNT uses future which does not come in standard in recent Python 3
versions, leading to mypy failing in Python 3 mode. Adding an explicit
dependency on future solves the problem.

Reviewed By: tnfchris

Differential Revision: https://reviews.llvm.org/D112148

Author: Thomas Preud'homme

tox.ini

@@ -19,6 +19,7 @@ skip_install = true
 deps =
     mypy >= 0.910
     sqlalchemy-stubs
+    future
 commands =
     # No where close to passing yet, but nice to have.
     - mypy --install-types --non-interactive --junit-xml=junit-{envname}.xml --py2 --ignore-missing-imports lnt