HTTP endpoint class should not include Authorization headers in __str__ method

Logging Authorization headers leaks credentials into logs (for Bearer tokens, see https://www.rfc-editor.org/rfc/rfc6750.html#section-5.3, first item recommendation, "Safeguard bearer tokens"). Suggest converting base_headers into a dictionary comprehension that redacts 'Authorization' at sgqlc/endpoint/http.py:104, as in:

{k: v for k, v in self.base_headers.items() if k != "Authorization"}