Use workaround for typing.Self missing in older Python versions

Author: timopollmeier

gvm/http/core/headers.py

@@ -7,7 +7,10 @@
 """
 
 from dataclasses import dataclass
-from typing import Dict, Optional, Self
+from typing import Dict, Optional, TypeVar
+
+
+Self = TypeVar("Self", bound="ContentType")
 
 
 @dataclass

gvm/http/core/response.py

@@ -7,13 +7,16 @@
 """
 
 from dataclasses import dataclass
-from typing import Any, Dict, Optional, Self
+from typing import Any, Dict, Optional, TypeVar
 
 from requests import Response
 
 from gvm.http.core.headers import ContentType
 
 
+Self = TypeVar("Self", bound="HttpResponse")
+
+
 @dataclass
 class HttpResponse:
     """