Update _sas.pyi

Author: jonashaag

pandas/io/sas/_sas.pyi

@@ -3,3 +3,5 @@ from pandas.io.sas.sas7bdat import SAS7BDATReader
 class Parser:
     def __init__(self, parser: SAS7BDATReader) -> None: ...
     def read(self, nrows: int) -> None: ...
+
+def get_subheader_index(signature: bytes) -> int: ...