Dear Maintainer, I suspect this issue is caused by python3-pytest-pep8 package. Just dropping pep8 support for pytest makes your package buildable.
Kind Regards
Index: flask-gravatar-0.4.2/debian/control =================================================================== --- flask-gravatar-0.4.2.orig/debian/control +++ flask-gravatar-0.4.2/debian/control @@ -9,7 +9,7 @@ Build-Depends: debhelper-compat (= 12), python3-coverage, python3-flask, python3-pytest, - python3-pytest-pep8, +# python3-pytest-pep8, python3-setuptools, python3-pytest-cov Standards-Version: 4.4.0 Index: flask-gravatar-0.4.2/pytest.ini =================================================================== --- flask-gravatar-0.4.2.orig/pytest.ini +++ flask-gravatar-0.4.2/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