travis: Add python 3.7 test

We just need to lift the dist for that target to xenial - but not to
bionic because 3.4 is then no longer supported.

In xenial, mercurial is part of travis image. Additionally installing it
is not needed and actually breaks things. But things are more
complicated because hg is under /usr/local/bin, and kas used a built-in
PATH for tools that does not include this.

At this chance, remove "sudo: required" - no longer needed.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

.travis.yml

@@ -1,20 +1,15 @@
 language: python
+dist: xenial
 
 python:
   - "3.4"
   - "3.5"
   - "3.6"
-
-sudo: required
+  - "3.7"
 
 services:
   - docker
 
-addons:
-  apt:
-    packages:
-      - mercurial
-
 install:
   # install kas to have all kas dependencies:
   - pip install .
@@ -22,6 +17,8 @@ install:
   - pip install flake8 pycodestyle doc8 Pygments
   # test dependencies:
   - pip install -U pytest
+  # workaround, needed because kas uses a cleaned-up PATH internally
+  - sudo ln -s /usr/local/bin/hg /usr/bin/
 
 script:
   - scripts/checkcode.sh .