drop python3.6 support

python 3.6 reached end of life on 2021-12-23

Committed via https://github.com/asottile/all-repos
diff --git a/pre_commit_hooks/util.py b/pre_commit_hooks/util.py
index 402e33e..d6c90ae 100644
--- a/pre_commit_hooks/util.py
+++ b/pre_commit_hooks/util.py
@@ -1,20 +1,19 @@
+from __future__ import annotations
+
 import subprocess
 from typing import Any
-from typing import List
-from typing import Optional
-from typing import Set
 
 
 class CalledProcessError(RuntimeError):
     pass
 
 
-def added_files() -> Set[str]:
+def added_files() -> set[str]:
     cmd = ('git', 'diff', '--staged', '--name-only', '--diff-filter=A')
     return set(cmd_output(*cmd).splitlines())
 
 
-def cmd_output(*cmd: str, retcode: Optional[int] = 0, **kwargs: Any) -> str:
+def cmd_output(*cmd: str, retcode: int | None = 0, **kwargs: Any) -> str:
     kwargs.setdefault('stdout', subprocess.PIPE)
     kwargs.setdefault('stderr', subprocess.PIPE)
     proc = subprocess.Popen(cmd, **kwargs)
@@ -25,7 +24,7 @@
     return stdout
 
 
-def zsplit(s: str) -> List[str]:
+def zsplit(s: str) -> list[str]:
     s = s.strip('\0')
     if s:
         return s.split('\0')