fix: pytest not in PATH for 3.3

Author: Maxim Kochukov

.travis.yml

@@ -8,7 +8,5 @@ matrix:
     - python: 3.7
       dist: xenial
       sudo: true
-install:
-  - pip install pytest # No pytest in 3.3
 script:
-  - pytest
+  - python -m pytest