dev-requirements.txt: Pinning required pytest and pylint versions

These packages should never change without revalidation of whether
our test suite passes. A change to these versions should include
changes to the code to ensure it still passes tests.

dev-requirements.txt

 coverage == 4.4.0
 pep8
-pytest >= 3.1.0
+pytest == 3.7.1
+pylint == 2.1.1
 pytest-cov >= 2.5.0
 pytest-datafiles
 pytest-env