provide option to extend expiration of jwt token (#1068)

supply optional parameter for `create_jwt` method

github/MainClass.py

@@ -742,15 +742,18 @@ class GithubIntegration(object):
         self.integration_id = integration_id
         self.private_key = private_key
 
-    def create_jwt(self):
+    def create_jwt(self, expiration=60):
         """
-        Creates a signed JWT, valid for 60 seconds.
+        Creates a signed JWT, valid for 60 seconds by default.
+        The expiration can be extended beyond this, to a maximum of 600 seconds.
+
+        :param expiration: int
         :return:
         """
         now = int(time.time())
         payload = {
             "iat": now,
-            "exp": now + 60,
+            "exp": now + expiration,
             "iss": self.integration_id
         }
         encrypted = jwt.encode(