fix: import the typing names the annotations use

Dict, List and Optional were used in 30 annotations without an import;
from __future__ import annotations kept that from failing until
something resolved them (typing.get_type_hints raised NameError).

Closes #29

napalm_linux/linux.py

@@ -23,7 +23,7 @@ import logging
 import re
 import socket
 from shlex import quote as _shlex_quote
-from typing import Any
+from typing import Any, Dict, List, Optional
 
 from netmiko import ConnectHandler
 from netmiko.exceptions import (