Update jinja_example.py

Added python version check for enabling async mode.

examples/jinja_example/jinja_example.py

@@ -5,13 +5,19 @@ from sanic import Sanic
 from sanic import response
 from jinja2 import Environment, PackageLoader, select_autoescape
 
+import sys
+# Enabling async template execution which allows you to take advantage
+# of newer Python features requires Python 3.6 or later.
+enable_async = sys.version_info >= (3, 6)
+
+
 app = Sanic(__name__)
 
 # Load the template environment with async support
 template_env = Environment(
     loader=PackageLoader('jinja_example', 'templates'),
     autoescape=select_autoescape(['html', 'xml']),
-    enable_async=True
+    enable_async=enable_async
 )
 
 # Load the template from file