Made jinja2 import lazy in django.forms.

Benchmarking shows that about 22% of the startup time for a simple
django project was spent importing jinja2, which the project doesn't
use.

It's reasonable to make this import lazy. This will only affect
projects where jinja2 is installed but not used, but given the
prevalence of jinja2 that's likely to be many environments (e.g. if
Ansible is installed, or the global Python install is used).

django/forms/renderers.py

@@ -7,12 +7,6 @@ from django.template.loader import get_template
 from django.utils.functional import cached_property
 from django.utils.module_loading import import_string
 
-try:
-    from django.template.backends.jinja2 import Jinja2
-except ImportError:
-    def Jinja2(params):
-        raise ImportError("jinja2 isn't installed")
-
 ROOT = Path(__file__).parent
 
 
@@ -58,7 +52,10 @@ class Jinja2(EngineMixin, BaseRenderer):
     Load Jinja2 templates from the built-in widget templates in
     django/forms/jinja2 and from apps' 'jinja2' directory.
     """
-    backend = Jinja2
+    @cached_property
+    def backend(self):
+        from django.template.backends.jinja2 import Jinja2
+        return Jinja2
 
 
 class TemplatesSetting(BaseRenderer):