✅ modstool: Add Travis configuration

.travis.yml

@@ -0,0 +1,13 @@
+dist: xenial  # required for Python >= 3.7
+language: python
+python:
+  - "3.5"
+  - "3.6"
+  - "3.7"
+
+
+install:
+  - pip install -r requirements.txt
+
+script:
+  - pytest