ci: Have one config run in xenial to test against python3.5

Also, bump the travis env to bionic. This shouldn't matter at all
because all ci configs run inside a docker, but it does seem to fix a
bug. See
https://github.com/bitcoin/bitcoin/pull/19267#issuecomment-643630309

.travis.yml

@@ -27,7 +27,7 @@
 
 version: ~> 1.0
 
-dist: xenial
+dist: bionic
 os: linux
 language: minimal
 arch: amd64
@@ -131,7 +131,7 @@ jobs:
         FILE_ENV="./ci/test/00_setup_env_native_multiprocess.sh"
 
     - stage: test
-      name: 'x86_64 Linux  [GOAL: install]  [bionic]  [no wallet]'
+      name: 'x86_64 Linux  [GOAL: install]  [xenial]  [no wallet]'
       env: >-
         FILE_ENV="./ci/test/00_setup_env_native_nowallet.sh"
 

ci/test/00_setup_env_native_nowallet.sh

@@ -7,6 +7,7 @@
 export LC_ALL=C.UTF-8
 
 export CONTAINER_NAME=ci_native_nowallet
+export DOCKER_NAME_TAG=ubuntu:16.04  # Use xenial to have one config run the tests in python3.5, see doc/dependencies.md
 export PACKAGES="python3-zmq"
 export DEP_OPTS="NO_WALLET=1"
 export GOAL="install"