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>
diff --git a/.travis.yml b/.travis.yml
index ac997b0..777212e 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -40,7 +40,12 @@
 
     - name: Windows
       os: windows
+      before_install:
+        - choco install python3
       script:
+        - export
+        - PATH=$PATH:'C:\ProgramData\chocolatey\bin'
+        - python3 scripts/generate_psa_constants.py
         - scripts/windows_msbuild.bat v141 # Visual Studio 2017
 
 after_failure: