Make using session the default (#1566)

I have no idea why you would not want to use a session by default. If you make more than 1 request in your script you benefit by an order of magnitude on wait to reconnect.

Author: Matthew-Jenkins

atlassian/rest_client.py

@@ -78,7 +78,7 @@ def __init__(
         api_root: str = "rest/api",
         api_version: Union[str, int] = "latest",
         verify_ssl: bool = True,
-        session: Optional[requests.Session] = None,
+        session: Optional[requests.Session] = requests.Session(),
         oauth: Optional[dict] = None,
         oauth2: Optional[dict] = None,
         cookies: Optional[CookieJar] = None,