Enable travis

.travis.yml

diff --git a/.travis.yml b/.travis.yml
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+# we only CI the master, release branches, tags and PRs
+if: tag IS present OR type = pull_request OR ((branch = master OR branch =~ release-*) AND type = push)
+
+language: python
+# cache package wheels (1 cache per python version)
+cache: pip
+# newer python versions are available only on xenial (while some older only on trusty) Ubuntu distribution
+dist: xenial
+sudo: required
+
+env:
+  TOXENV=py
+
+jobs:
+  include:
+  - name: "run test suite with python 3.4"
+    python: 3.4
+    dist: trusty
+  - name: "run test suite with python 3.5"
+    python: 3.5.1
+    dist: trusty
+  - name: "run test suite with python 3.6"
+    python: 3.6
+  - name: "run test suite with python 3.7"
+    python: 3.7
+  - name: "run test suite with python 3.8-dev"
+    python: 3.8-dev
+  - name: "check code style with flake8"
+    python: 3.7
+    env:
+    - TOXENV=lint
+
+install:
+- pip install -U pip setuptools
+- pip install -U tox
+- tox --notest
+
+script:
+- tox