Remove unnecessary Python version check (#1305)

Summary:
Pull Request resolved: #1305

As title. The check is no longer needed after we bumped min Python version to 3.8.

Reviewed By: aelavender

Differential Revision: D59181986

fbshipit-source-id: 0e58ab9b002ddcb6fe8ecabe16f0edaa0f4fa67f

captum/_utils/typing.py

@@ -6,12 +6,7 @@
 from torch.nn import Module
 
 if TYPE_CHECKING:
-    import sys
-
-    if sys.version_info >= (3, 8):
-        from typing import Literal  # noqa: F401
-    else:
-        from typing_extensions import Literal  # noqa: F401
+    from typing import Literal
 else:
     Literal = {True: bool, False: bool, (True, False): bool}