fix py2.7

.travis.yml

@@ -17,7 +17,7 @@ before_install:
   - export CC="gcc-4.9"
   - export CXX="g++-4.9"
 install:
-  - pip install future
+  - pip install future; python_version < '3.0'
   - pip install numpy
   - pip install torch_nightly -f https://download.pytorch.org/whl/nightly/cpu/torch_nightly.html
   - pip install pycodestyle

setup.py

@@ -25,7 +25,11 @@ url = 'https://github.com/rusty1s/pytorch_scatter'
 
 install_requires = []
 setup_requires = ['pytest-runner']
-tests_require = ['pytest', 'pytest-cov']
+tests_require = [
+    'pytest<5.0;python_version<"3.0"',
+    'pytest;python_version>="3.0"',
+    'pytest-cov',
+]
 
 setup(
     name='torch_scatter',