Require a sufficiently recent version of pytest

.travis.yml

@@ -7,7 +7,6 @@ python:
   - "3.6"
   - pypy
 install:
-  - pip install --upgrade pytest
   - pip install -r requirements-travis.txt
   - pip install -e .
 script: pytest

requirements-dev.txt

@@ -1,9 +1,9 @@
 -r requirements-travis.txt
+
 # test tools
-pytest
 tox
 
 # documentation
-Pygments>=1.6
+Pygments >= 1.6
 Sphinx
 sphinx_rtd_theme

requirements-travis.txt

@@ -1,3 +1,5 @@
+pytest >= 3.2.1
+
 # code quality
 flake8
 coverage