fix: str to bytes conversion

Author: pacesm

autotest/autotest_services/base.py

@@ -98,7 +98,7 @@ def _get_content(response):
 def pack_multipart_request(parts, boundary):
 
     def _format_header(key, value):
-        return b(f"{key}: {value}")
+        return (f"{key}: {value}").encode("ascii")
 
     def _generate_multipart_request(parts):
         yield b""