Cast GitHub app_id as string before using as iss

The pyjwt library expects the issuer to be of type `str` and enforces it
with jpadilla/pyjwt#1040. Using an integer as
done before will result in `TypeError: Issuer (iss) must be a string.`.

Signed-off-by: Jonas Brand (8R0WNI3) <j.brand@sap.com>

Author: 8R0WNI3

src/github3/apps.py

@@ -174,7 +174,7 @@ def create_token(private_key_pem, app_id, expire_in=TEN_MINUTES_AS_SECONDS):
         raise ValueError('"private_key_pem" parameter must be byte-string')
     now = int(time.time())
     token = jwt.encode(
-        payload={"iat": now, "exp": now + expire_in, "iss": app_id},
+        payload={"iat": now, "exp": now + expire_in, "iss": str(app_id)},
         key=private_key_pem,
         algorithm="RS256",
     )