Fixbug Type error a bytes-like (#1204)

Added encode 'utf-8' for code_verifier

Author: islam-kamel

docs/getting_started.rst

@@ -266,7 +266,7 @@ Now let's generate an authentication code grant with PKCE (Proof Key for Code Ex
     import hashlib
 
     code_verifier = ''.join(random.choice(string.ascii_uppercase + string.digits) for _ in range(random.randint(43, 128)))
-    code_verifier = base64.urlsafe_b64encode(code_verifier)
+    code_verifier = base64.urlsafe_b64encode(code_verifier.encode('utf-8'))
 
     code_challenge = hashlib.sha256(code_verifier.encode('utf-8')).digest()
     code_challenge = base64.urlsafe_b64encode(code_challenge).decode('utf-8').replace('=', '')