Use Python3.5 instead of Python3.4 on Debian 9
diff --git a/templates/tools/dockerfile/test/python_pyenv_x64/Dockerfile.template b/templates/tools/dockerfile/test/python_pyenv_x64/Dockerfile.template
index b408b74..1e013b7 100644
--- a/templates/tools/dockerfile/test/python_pyenv_x64/Dockerfile.template
+++ b/templates/tools/dockerfile/test/python_pyenv_x64/Dockerfile.template
@@ -20,9 +20,9 @@
   <%include file="../../gcp_api_libraries.include"/>
   <%include file="../../python_deps.include"/>
   <%include file="../../apt_get_pyenv.include"/>
-  # Install pip and virtualenv for Python 3.4
-  RUN curl https://bootstrap.pypa.io/get-pip.py | python3.4
-  RUN python3.4 -m pip install virtualenv
+  # Install pip and virtualenv for Python 3.5
+  RUN curl https://bootstrap.pypa.io/get-pip.py | python3.5
+  RUN python3.5 -m pip install virtualenv
 
   <%include file="../../run_tests_addons.include"/>
   # Define the default command.