Travis-CI: Make using python3 explicit

Author: mgeier

.travis.yml

@@ -11,13 +11,15 @@ addons:
       - pandoc
       - ffmpeg
 install:
-  - pip install .
-  - pip install -r tests/requirements.txt
-  - pip install -r doc/requirements.txt
+  - python3 --version
+  - python3 -m pip --version
+  - python3 -m pip install .
+  - python3 -m pip install -r tests/requirements.txt
+  - python3 -m pip install -r doc/requirements.txt
   # This is needed in example scripts:
-  - pip install pillow
+  - python3 -m pip install pillow
 script:
-  - python -m pytest
-  - python doc/examples/run_all.py
+  - python3 -m pytest
+  - python3 doc/examples/run_all.py
   # This executes the example notebooks and runs the doctests:
-  - python -m sphinx doc/ _build/ -b doctest
+  - python3 -m sphinx doc/ _build/ -b doctest