tests: disable pytest-flake8 on Python 2.6

Python 2.6 support has been official dropped from
flake8 for some time, and broken since 3.4.0.

tests/requirements.txt

 importlib; python_version<"2.7"
 mock
-pytest-flake8
+pytest-flake8; python_version>="2.7"
 pytest-virtualenv>=1.2.7
 pytest>=3.0.2