1. I didn't see client_id is optional in the doc.
2. You are always passing client_id=None

1. I didn't see client_id is optional in the doc.
2. You are always passing client_id=None

1. From linked section: "Authentication of the client is optional, as described in
   Section 3.2.1 of OAuth 2.0 [RFC6749], and consequently, the
   "client_id" is only needed when a form of client authentication that
   relies on the parameter is used."

One of the providers I'm working with uses the authorization grant routine with client id.

2. Good catch, fixed

I would guess client_id is indeed optional:

RFC7521 section 4.1:

Authentication of the client is optional, as described in
Section 3.2.1 of OAuth 2.0 [RFC6749], and consequently, the
"client_id" is only needed when a form of client authentication that
relies on the parameter is used.

RFC6749 section 3.2.1

A client MAY use the "client_id" request parameter to identify itself
when sending requests to the token endpoint.