Fix for Python 3.6: Callable is in typing, not typing_extensions

Even back in Python 3.6, typing had Callable in it, so this
should be here.

We need to rejig this *again* to do everything differently for
3.9+ as a bunch of these are 'deprecated' from 3.9 onwards, but
that's another topic.

Signed-off-by: Adam Williamson <[email protected]>

Author: AdamWill

src/openqa_client/client.py

@@ -25,7 +25,7 @@
 import logging
 import time
 import sys
-from typing import Any, Dict, List, MutableMapping, NoReturn, Optional, Union, overload
+from typing import Any, Callable, Dict, List, MutableMapping, NoReturn, Optional, Union, overload
 
 from urllib.parse import urlparse, urlunparse
 import configparser
@@ -35,9 +35,9 @@
 import openqa_client.exceptions
 
 if sys.version_info >= (3, 8):
-    from typing import Literal, TypedDict, Callable
+    from typing import Literal, TypedDict
 else:  # pragma: no cover
-    from typing_extensions import Literal, TypedDict, Callable  # pragma: no cover
+    from typing_extensions import Literal, TypedDict  # pragma: no cover
 
 
 logger = logging.getLogger(__name__)