build: don't show encoding warnings from other tools

Author: nedbat

tox.ini

@@ -72,8 +72,14 @@ basepython = python3.11
 # return.
 deps =
     -r doc/requirements.pip
+
 allowlist_externals =
     make
+
+setenv =
+    {[testenv]setenv}
+    PYTHONWARNDEFAULTENCODING=
+
 commands =
     # If cog fails, it means the docs need to be updated. Run `make prebuild`.
     python -m cogapp -cP --check --check-fail-msg='run `make prebuild`' --verbosity=1 doc/*.rst doc/*/*.rst
@@ -93,6 +99,7 @@ deps =
 
 setenv =
     {[testenv]setenv}
+    PYTHONWARNDEFAULTENCODING=
     LINTABLE=coverage tests doc ci igor.py setup.py __main__.py
 
 commands =
@@ -117,6 +124,7 @@ deps =
 
 setenv =
     {[testenv]setenv}
+    PYTHONWARNDEFAULTENCODING=
     TYPEABLE=coverage tests
 
 commands =