travis: use a matrix to allow for more languages

.travis.yml

@@ -1,13 +1,34 @@
 ---
-language: python
-python:
-  - '3.6'
-  - '3.7'
-install:
+# use a matrix to support multiple languages
+matrix:
+    include:
+      - language: python
+        python: '3.6'
+        install:
           - pip install -r requirements.txt
           - pip install -r requirements_develop.txt
           - pip install .
-script:
+        script:
+          - pytest --cov=simplezfs --cov-report=term-missing ${@}
+          - mypy src/simplezfs
+          - flake8
+      - language: python
+        python: '3.7'
+        install:
+          - pip install -r requirements.txt
+          - pip install -r requirements_develop.txt
+          - pip install .
+        script:
+          - pytest --cov=simplezfs --cov-report=term-missing ${@}
+          - mypy src/simplezfs
+          - flake8
+      - language: python
+        python: '3.7'
+        install:
+          - pip install -r requirements.txt
+          - pip install -r requirements_develop.txt
+          - pip install .
+        script:
           - pytest --cov=simplezfs --cov-report=term-missing ${@}
           - mypy src/simplezfs
           - flake8