Travis: Windows: install Python 3 and run generate_psa_constants.py

Travis Windows machines currently don't have Python 3 preinstalled.
We need it to run scripts/generate_psa_constants.py which is needed
to build mbedTLS.sln.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

.travis.yml

@@ -40,7 +40,16 @@ jobs:
 
     - name: Windows
       os: windows
+      before_install:
+        - choco install python --version=3.5.4
+      env:
+        # Add the directory where the Choco package goes
+        - PATH=/c/Python35:/c/Python35/Scripts:$PATH
       script:
+        - type python; python --version
+        - python scripts/generate_psa_constants.py
+        # Logs appear out of sequence on Windows. Give time to catch up.
+        - sleep 5
         - scripts/windows_msbuild.bat v141 # Visual Studio 2017
 
 after_failure: