Enable the opt-in EncodingWarning

https://docs.python.org/3/library/io.html#io-encoding-warning

tox.ini

@@ -28,7 +28,7 @@ setenv =
     PYTHONWARNINGS = all
     PYTEST_ADDOPTS = {env:PYTEST_ADDOPTS:} --color yes
 commands=
-    python -X dev -m pytest --durations 25 {posargs}
+    python -X dev -X warn_default_encoding -m pytest --durations 25 {posargs}
 
 [testenv:du-latest]
 commands =
@@ -83,7 +83,7 @@ description =
 extras =
     docs
 commands =
-    python setup.py build_sphinx {posargs}
+    python -X dev -X warn_default_encoding setup.py build_sphinx {posargs}
 
 [testenv:docslint]
 basepython = python3