Change Python Windows tests to 3.5

tools/dockerfile/test/python_jessie_x64/Dockerfile

@@ -75,7 +75,8 @@ RUN pip install --upgrade google-api-python-client
 RUN apt-get update && apt-get install -y \
     python-all-dev \
     python3-all-dev \
-    python-pip
+    python-pip \
+    python3-pip
 
 # Install Python packages from PyPI
 RUN pip install pip --upgrade

tools/run_tests/helper_scripts/build_python.sh

@@ -146,6 +146,7 @@ fi
 ############################
 # Perform build operations #
 ############################
+$PYTHON -m pip install virtualenv
 
 # Instnatiate the virtualenv, preferring to do so from the relevant python
 # version. Even if these commands fail (e.g. on Windows due to name conflicts)

tools/run_tests/run_tests.py

@@ -672,7 +672,7 @@ class PythonLanguage(object):
 
     if args.compiler == 'default':
       if os.name == 'nt':
-        return (python27_config,)
+        return (python35_config,)
       else:
         return (python27_config, python34_config,)
     elif args.compiler == 'python2.7':