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/check_docstring_first.py b/pre_commit_hooks/check_docstring_first.py
index 875c0fb..1744919 100644
--- a/pre_commit_hooks/check_docstring_first.py
+++ b/pre_commit_hooks/check_docstring_first.py
@@ -1,8 +1,9 @@
+from __future__ import annotations
+
 import argparse
 import io
 import tokenize
 from tokenize import tokenize as tokenize_tokenize
-from typing import Optional
 from typing import Sequence
 
 NON_CODE_TOKENS = frozenset((
@@ -45,7 +46,7 @@
     return 0
 
 
-def main(argv: Optional[Sequence[str]] = None) -> int:
+def main(argv: Sequence[str] | None = None) -> int:
     parser = argparse.ArgumentParser()
     parser.add_argument('filenames', nargs='*')
     args = parser.parse_args(argv)