Only import jinja2 when needed

changelog.d/5513.bugfix

@@ -0,0 +1 @@
+Prevent python module `jinja2` from being required even though it is marked as an optional dependency.

synapse/rest/client/v2_alpha/account.py

@@ -15,11 +15,10 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 import logging
+import sys
 
 from six.moves import http_client
 
-import jinja2
-
 from twisted.internet import defer
 
 from synapse.api.constants import LoginType
@@ -309,6 +308,9 @@ def load_jinja2_template(self, template_dir, template_filename, template_vars):
         Returns:
             str containing the contents of the rendered template
         """
+        if "jinja2" not in sys.modules:
+            import jinja2
+
         loader = jinja2.FileSystemLoader(template_dir)
         env = jinja2.Environment(loader=loader)