Travis CI: Add Python 3.7 and pypy3

.travis.yml

@@ -1,13 +1,12 @@
 language: python
 python:
+  - "pypy3"
   - "pypy"
   - "3.6"
   - "3.5"
   - "3.4"
   - "2.7"
 
-sudo: false
-
 cache: pip
 
 env:
@@ -18,6 +17,18 @@ env:
     - TOXENV=base
     - TOXENV=six19-optional
 
+matrix:
+  include:
+    - python: "3.7"
+      dist: xenial  # required for Python >= 3.7
+      env: TOXENV=optional
+    - python: "3.7"
+      dist: xenial  # required for Python >= 3.7
+      env: TOXENV=base
+    - python: "3.7"
+      dist: xenial  # required for Python >= 3.7
+      env: TOXENV=six19-optional
+
 install:
   - pip install tox codecov
 

requirements-optional.txt

@@ -14,4 +14,6 @@ lxml ; platform_python_implementation == 'CPython'
 
 # DATrie can be used in place of our Python trie implementation for
 # slightly better parsing performance.
-datrie ; platform_python_implementation == 'CPython'
+# https://github.com/pytries/datrie/issues/52 although closed is not
+# yet released to https://pypi.org/project/datrie
+datrie ; platform_python_implementation == 'CPython' and python_version < '3.7'