Enable PYTHONWARNDEFAULTENCODING to catch an encoding mistake.

(These files are all ASCII AFAIR, so probably it didn't matter much,
this line came more or less from jsonschema where I don't think anyone
reported a concrete issue, but nevertheless, it's wrong.)

Closes: python-jsonschema/jsonschema#1107

jsonschema_specifications/_core.py

@@ -28,5 +28,5 @@ def _schemas():
         for child in version.iterdir():
             children = [child] if child.is_file() else child.iterdir()
             for path in children:
-                contents = json.loads(path.read_text())
+                contents = json.loads(path.read_text(encoding="utf-8"))
                 yield Resource.from_contents(contents)

noxfile.py

@@ -1,4 +1,5 @@
 from pathlib import Path
+import os
 
 import nox
 
@@ -23,7 +24,8 @@ def _session(fn):
 @session(python=["3.8", "3.9", "3.10", "3.11", "pypy3"])
 def tests(session):
     session.install("pytest", ROOT)
-    session.run("pytest", "--verbosity=3")
+    env = dict(os.environ, PYTHONWARNDEFAULTENCODING="1")
+    session.run("pytest", "--verbosity=3", "--pythonwarnings=error", env=env)
 
 
 @session()