[2.0.x] Fixed #29617 -- Fixed Template crash if template_string is lazy.

Regression in 3a148f958d.
Backport of 9f3b9ffd51 from master.

django/template/base.py

@@ -156,7 +156,7 @@ class Template:
         self.name = name
         self.origin = origin
         self.engine = engine
-        self.source = template_string
+        self.source = str(template_string)  # May be lazy.
         self.nodelist = self.compile_nodelist()
 
     def __iter__(self):

docs/releases/2.0.8.txt

@@ -11,3 +11,6 @@ Bugfixes
 
 * Fixed a regression in Django 2.0.7 that broke the ``regex`` lookup on MariaDB
   (even though MariaDB isn't officially supported) (:ticket:`29544`).
+
+* Fixed a regression where ``django.template.Template`` crashed if the
+  ``template_string`` argument is lazy (:ticket:`29617`).

tests/template_tests/test_base.py

@@ -1,5 +1,12 @@
-from django.template.base import VariableDoesNotExist
+from django.template import Context, Template, VariableDoesNotExist
 from django.test import SimpleTestCase
+from django.utils.translation import gettext_lazy
+
+
+class TemplateTests(SimpleTestCase):
+    def test_lazy_template_string(self):
+        template_string = gettext_lazy('lazy string')
+        self.assertEqual(Template(template_string).render(Context()), template_string)
 
 
 class VariableDoesNotExistTests(SimpleTestCase):