Dev requirements: range for pytest

Author: Codeslayer666

.travis.yml

@@ -7,7 +7,7 @@ python:
   - "3.6"
   - "nightly"
 install:
-  - pip install tox-travis
+  - pip install tox tox-travis
   - pip install -r requirements.txt
 script:
   - tox

requirements-dev.txt

@@ -1,4 +1,4 @@
-pytest==4.1
+pytest>=2.9.2,<4.1
 py>=1.4.26
 mock>=1.0.1
 responses==0.3.0