stretch, zesty, and xenial do not have python(3)-pytest-pep8, so remove the
test on these distributions.

(This patch is commented out by default in debian/patches/series.)

--- a/pytest.ini
+++ b/pytest.ini
@@ -9,4 +9,4 @@
 
 
 [pytest]
-addopts = --pep8 --ignore=docs --cov=flask_gravatar --cov-report=term-missing tests flask_gravatar
+addopts = --ignore=docs --cov=flask_gravatar --cov-report=term-missing tests flask_gravatar
