Travis CI: install typing on python2 to restore some skipped tests (#9528)

This used to be installed but was removed in def2c63990dd#diff-354f30a63fb0907d4ad57269548329e3L15
in `mypy/util.py:try_find_python2_interpreter()` we try to find a valid python2 interpreter with typing installed.
If we don't find it, tests are skipped.
However on python/typeshed, the CI installs typing on python2, and thus doesn't skip the tests
Resolves #9473

Author: vbarbaresi

.travis.yml

@@ -85,6 +85,7 @@ install:
 - pip install -U pip setuptools
 - pip install -U 'virtualenv<20'
 - pip install -U tox==3.9.0
+- python2 -m pip install --user -U typing
 - tox --notest
 
 # This is a big hack and only works because the layout of our directories