Fix dist install test by ensuring that we use Python 3 (#9011)

* Fix dist install test by ensuring that we use Python 3

Now that we have dropped Python 2 support, we need to make sure this
install test uses Python 3.

* Update Docker image to install Python 3 version of setuptools

* Run pip3 instead of pip
diff --git a/kokoro/linux/dockerfile/test/java_stretch/Dockerfile b/kokoro/linux/dockerfile/test/java_stretch/Dockerfile
index 3e72046..b9f562a 100644
--- a/kokoro/linux/dockerfile/test/java_stretch/Dockerfile
+++ b/kokoro/linux/dockerfile/test/java_stretch/Dockerfile
@@ -24,7 +24,7 @@
   maven \
   openjdk-8-jdk \
   # Python dependencies
-  python-setuptools \
-  python-pip \
+  python3-setuptools \
+  python3-pip \
   virtualenv \
   && apt-get clean
diff --git a/tests.sh b/tests.sh
index 1955b7b..5dc2eb6 100755
--- a/tests.sh
+++ b/tests.sh
@@ -112,8 +112,8 @@
   virtualenv --no-site-packages venv
   source venv/bin/activate
   pushd python
-  python setup.py clean build sdist
-  pip install dist/protobuf-*.tar.gz
+  python3 setup.py clean build sdist
+  pip3 install dist/protobuf-*.tar.gz
   popd
   deactivate
   rm -rf python/venv