Make django_settings_is_configured return the first value always

I have been seeing an issue, where it would return False initially, but
later True; causing an `AttributeError` with the `mailoutbox` fixture,
because `mail.outbox` was not set on the module.

This can happen with imported code that changes the environment, e.g.
via "wsgi/asgi.py":

> os.environ.setdefault("DJANGO_SETTINGS_MODULE", "foo.settings")

Author: blueyed

pytest_django/lazy_django.py

@@ -8,6 +8,9 @@
 import pytest
 
 
+_cache = {}
+
+
 def skip_if_no_django():
     """Raises a skip exception when no Django settings are available"""
     if not django_settings_is_configured():
@@ -21,10 +24,15 @@ def django_settings_is_configured():
     configured flag in the Django settings object if django.conf has already
     been imported.
     """
-    ret = bool(os.environ.get("DJANGO_SETTINGS_MODULE"))
+    try:
+        return _cache["django_is_configured"]
+    except KeyError:
+        ret = bool(os.environ.get("DJANGO_SETTINGS_MODULE"))
+
+        if not ret and "django.conf" in sys.modules:
+            ret = sys.modules["django.conf"].settings.configured
 
-    if not ret and "django.conf" in sys.modules:
-        return sys.modules["django.conf"].settings.configured
+        _cache["django_is_configured"] = ret
 
     return ret