travis: lets try with python 3

Author: nunoplopes

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=python
+ARG PYTHON_EXECUTABLE=/usr/bin/python3
 ARG RUN_API_EXAMPLES
 ARG RUN_SYSTEM_TESTS
 ARG RUN_UNIT_TESTS