Run mypy on Travis

`tests/scripts/all.sh check_python_files` now runs mypy (in addition
to pylint) if it's available. So install mypy.

Install mypy 0.780, which is the earliest version that works on our
code at this time.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/.travis.yml b/.travis.yml
index 76cb1c5..9b729ec 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -17,7 +17,7 @@
       language: python # Needed to get pip for Python 3
       python: 3.5 # version from Ubuntu 16.04
       install:
-        - pip install pylint==2.4.4
+        - pip install mypy==0.780 pylint==2.4.4
       script:
         - tests/scripts/all.sh -k 'check_*'
         - tests/scripts/all.sh -k test_default_out_of_box