Add a method for making GraphQL requests

[jwodder]

• Give Client (but not Endpoint) the following method:

  def graphql(
      self,
      query: str,
      variables: dict[str, Any] | None = None,
      *,
      headers: HeadersType = None,
      timeout: TimeoutType = None,
      raw: bool = False,
  ) -> Any

  This method makes a POST request to f"{api_url}/graphql". Unless a different value (possibly None) is set in headers, the request's headers will include "X-Github-Next-Global-ID: 1".

  • Don't set the X-GitHub-Api-Version header?
  • Don't set the Accept header?

• graphql() should use (almost?) the same retrying logic as the main API code.

• graphql() should not insert a mutation delay between POST requests.

• Use this method in reponews.