contrib: print the version of pytype used to do the type checking

This will help with CI.  I don't see a way to print the version of python that's
running it.  When I tried `head -n 1 $(which pytype)`, the CI run printed:

    #!/usr/bin/env bash

Locally, that gives the path to the python interpreter in the venv, so IDK
what's different.

contrib/check-pytype.sh

@@ -5,6 +5,9 b' set -u'
 
 cd "$(hg root)"
 
+printf "pytype version: "
+pytype --version
+
 # Many of the individual files that are excluded here confuse pytype
 # because they do a mix of Python 2 and Python 3 things
 # conditionally. There's no good way to help it out with that as far as