[travis] Add python 3.7 build

.travis.yml

@@ -15,6 +15,12 @@ env:
   - YTDL_TEST_SET=download
 matrix:
   include:
+    - python: 3.7
+      dist: xenial
+      env: YTDL_TEST_SET=core
+    - python: 3.7
+      dist: xenial
+      env: YTDL_TEST_SET=download
     - env: JYTHON=true; YTDL_TEST_SET=core
     - env: JYTHON=true; YTDL_TEST_SET=download
   fast_finish: true