travis: fall back to python 2; will move to 3 with ubuntu 20 only

Author: nunoplopes

contrib/ci/Dockerfiles/z3_base_ubuntu_18.04.Dockerfile

@@ -14,6 +14,8 @@ RUN apt-get update && \
         graphviz \
         python3 \
         python3-setuptools \
+        python2.7 \
+        python-setuptools \
         sudo
 
 RUN curl -SL https://packages.microsoft.com/config/ubuntu/16.04/packages-microsoft-prod.deb --output packages-microsoft-prod.deb && \

contrib/ci/Dockerfiles/z3_build.Dockerfile

@@ -12,7 +12,7 @@ ARG DOTNET_BINDINGS
 ARG JAVA_BINDINGS
 ARG NO_SUPPRESS_OUTPUT
 ARG PYTHON_BINDINGS
-ARG PYTHON_EXECUTABLE=/usr/bin/python3
+ARG PYTHON_EXECUTABLE=/usr/bin/python
 ARG RUN_API_EXAMPLES
 ARG RUN_SYSTEM_TESTS
 ARG RUN_UNIT_TESTS