style(tests): Fix new flake8 errors

This change fixes new errors detected by the last version of pycodestyle
(2.2.0), which is a dependency of flake8:

    ./tests/test_spec_examples.py:51:1: E305 expected 2 blank lines
    after class or function definition, found 1
    ./tests/test_spec_examples.py:139:1: E305 expected 2 blank lines
    after class or function definition, found 1

See pycodestyle changelog at 2.2.0 and
https://github.com/PyCQA/pycodestyle/pull/593.

tests/test_spec_examples.py

@@ -48,6 +48,7 @@ from tests.common import RuleTestCase
 class SpecificationTestCase(RuleTestCase):
     rule_id = None
 
+
 conf_general = ('document-start: disable\n'
                 'comments: {min-spaces-from-content: 1}\n'
                 'braces: {min-spaces-inside: 1, max-spaces-inside: 1}\n'
@@ -133,6 +134,7 @@ def _gen_test(buffer, conf):
         self.check(buffer, conf)
     return test
 
+
 # The following tests are blacklisted (i.e. will not be checked against
 # yamllint), because pyyaml is currently not able to parse the contents
 # (using yaml.parse()).