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

kokoro/linux/dockerfile/test/java_stretch/Dockerfile

@@ -24,7 +24,7 @@ RUN apt-get update && apt-get install -y \
   maven \
   openjdk-8-jdk \
   # Python dependencies
-  python-setuptools \
-  python-pip \
+  python3-setuptools \
+  python3-pip \
   virtualenv \
   && apt-get clean

tests.sh

@@ -112,8 +112,8 @@ build_dist_install() {
   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