hghave: add a check for pytype, Google's Python type checker I'm not unbiased, but I strongly prefer pytype to mypy: it does more inference, so we don't need as many annontations. In theory we can use both, but since I'm doing some tinkering with pytype I figured I'd just try setting it up. Differential Revision: https://phab.mercurial-scm.org/D7268