core: TokenAuth request_token fix missing auth

the method is intended to request authenticated
token, per pydocs, but was passing an headers
which was always missing Authorization.

Signed-off-by: tarilabs <matteo.mortari@gmail.com>

oras/auth/token.py

@@ -100,6 +100,8 @@ def request_token(self, h: auth_utils.authHeader) -> bool:
         """
         params = {}
         headers = {}
+        if self._basic_auth: # we exchange the basic auth for the token
+            headers["Authorization"] = "Basic %s" % self._basic_auth
 
         # Prepare request to retry
         if h.service: