Remove Python 3.6 branches from typeshed (#8269)

stdlib/struct.pyi

@@ -1,4 +1,3 @@
-import sys
 from _typeshed import ReadableBuffer, WriteableBuffer
 from collections.abc import Iterator
 from typing import Any
@@ -15,10 +14,7 @@ def iter_unpack(__format: str | bytes, __buffer: ReadableBuffer) -> Iterator[tup
 def calcsize(__format: str | bytes) -> int: ...
 
 class Struct:
-    if sys.version_info >= (3, 7):
-        format: str
-    else:
-        format: bytes
+    format: str
     size: int
     def __init__(self, format: str | bytes) -> None: ...
     def pack(self, *v: Any) -> bytes: ...