Extended travis config

- Python versions added (3.9, pypy)
- OS added (MacOS, 2 different versions)

.travis.yml

@@ -1,11 +1,45 @@
 language: python
+os: linux
+cache: pip
 
-python:
-  - 2.7
-  - 3.5
-  - 3.6
-  - 3.7
-  - 3.8
+matrix:
+  include:
+    - name: "Python 2.7 on Linux"
+      python: 2.7
+    - name: "Python 3.5 on Linux"
+      python: 3.5
+    - name: "Python 3.6 on Linux"
+      python: 3.6
+    - name: "Python 3.7 on Linux"
+      python: 3.7
+    - name: "Python 3.8 on Linux"
+      dist: xenial
+      python: 3.8
+    - name: "Python 3.9 Nightly on Linux"
+      dist: bionic
+      python: nightly
+    - name: "Pypy 3 on Linux"
+      python: pypy3
+    - name: "Python 3 on older macOS"
+      os: osx
+      osx_image: xcode9.4
+      language: shell
+      before_install:
+        - sw_vers
+        - python3 --version
+        - pip3 --version
+    - name: "Python 3 on macOS"
+      os: osx
+      osx_image: xcode11
+      language: shell
+      before_install:
+        - sw_vers
+        - python3 --version
+        - pip3 --version
+  allow_failures:
+    - python: nightly
+    - python: pypy3
+    - os: osx
 
 install:
   - travis_retry pip install -U pip wheel tox-travis