Run tests with warnings as errors.

Author: domdfcoding

tox.ini

@@ -24,6 +24,7 @@ envlist =
     py311-dev
     pypy36
     pypy37
+    pypy38
     mypy
     build
 skip_missing_interpreters = True
@@ -149,6 +150,9 @@ package = flake8_sphinx_links
 [pytest]
 addopts = --color yes --durations 25
 timeout = 300
+filterwarnings =
+    error
+    ignore:can't resolve package from __spec__ or __package__, falling back on __name__ and __path__:ImportWarning
 
 [testenv:lint]
 basepython = python3.6