Avoid using __file__ at module level in django.forms.renderers

The module-level constant `ROOT` is used only in one place. The method it's used
in is already a cached property, so there shouldn't be a real performance issue.
`ROOT` is undocumented, so I'm assuming it's an obscure private symbol that no
one will miss.

django/forms/renderers.py

@@ -7,8 +7,6 @@
 from django.utils.functional import cached_property
 from django.utils.module_loading import import_string
 
-ROOT = Path(__file__).parent
-
 
 @functools.lru_cache()
 def get_default_renderer():
@@ -33,7 +31,7 @@ def get_template(self, template_name):
     def engine(self):
         return self.backend({
             'APP_DIRS': True,
-            'DIRS': [ROOT / self.backend.app_dirname],
+            'DIRS': [Path(__file__).parent / self.backend.app_dirname],
             'NAME': 'djangoforms',
             'OPTIONS': {},
         })