mypy: skip following _pytest imports

Newer pytest versions contain variable annotations, and they fail as
we're running mypy with python_version = 3.4.

Author: scop

test/setup.cfg

@@ -9,6 +9,10 @@ python_version = 3.4
 show_error_codes = true
 ignore_missing_imports = true
 
+[mypy-_pytest.*]
+# variable annotations in newer pytest, errors out with python_version < 3.6
+follow_imports = skip
+
 [isort]
 known_first_party = conftest
 known_third_party = pexpect,pytest