Freeze pytest version requirement to 3.6.4

The 3.7.x releases seem to suffer from a regression in tests collection
that affects our linting process. It may have been fixed upstream
already [1]. Freezing requirement to latest 3.6.x for now.

[1] pytest-dev/pytest#3802

.pylintrc

@@ -7,7 +7,7 @@ extension-pkg-whitelist=
 
 # Add files or directories to the blacklist. They should be base names, not
 # paths.
-#ignore=CVS,tests,doc
+#ignore=tests,docs
 
 # Add files or directories matching the regex patterns to the blacklist. The
 # regex matches against base names, not paths.

setup.py

@@ -89,7 +89,7 @@ def get_cmdclass():
     'coverage == 4.4.0',
     'moto',
     'pep8',
-    'pytest >= 3.1.0',
+    'pytest == 3.6.4',
     'pytest-cov >= 2.5.0',
     'pytest-pep8',
     'pytest-pylint',