Mark test_validate_doi as flaky

Some resolvers abuse HTTP 403 to mean HTTP 429 (Too Many Requests) and
therefore make the test more flaky. Rerunning usually avoids this issue.

Author: hseg

setup.py

@@ -22,6 +22,7 @@ def get_version(filename):
 
 dev_requirements = [
     'coverage', 'pytest', 'pytest-cov==2.5.0', 'twine', 'pep8',
+    'pytest-rerunfailures',
     'flake8', 'wheel', 'mypy',
     'sphinx', 'sphinx-autobuild', 'sphinx-autodoc-typehints',
     'sphinx_rtd_theme']

tests/test_doi.py

@@ -45,6 +45,7 @@ def test_valid_version() -> None:
 
 
 @pytest.mark.net
+@pytest.mark.flaky(reruns=5, reruns_delay=1, rerun_except="HTTPError")
 def test_validate_doi() -> None:
     data = [
         ('10.1063/1.5081715',