Change recommendation from python3 setup.py test to pytest

CONTRIBUTING.md

@@ -31,7 +31,7 @@
   - Create a new local branch: `git checkout -b branchname`
   - Make any changes in it
   - Make sure that your code still conforms to the formatting standard: `black -t py36 .`
-  - Run the test suite over it to expose any problems: `python3 setup.py test`
+  - Run the test suite over it to expose any problems: `pytest`
   - Push the local branch to your fork on github: `git push -u origin branchname`
   - Go to github.com/yourname/moviepy and it will display 'Recently pushed branches' giving you the option to make a Pull Request to the main repo
   - Fill in any details for your PR and submit