Update timestamps.pyi

pandas-dev/pandas@f3c46cd

pandas-dev/pandas#48819

pandas-dev#797

Author: davetapley

pandas-stubs/_libs/tslibs/timestamps.pyi

@@ -301,3 +301,6 @@ class Timestamp(datetime):
     def days_in_month(self) -> int: ...
     @property
     def daysinmonth(self) -> int: ...
+    @property
+    def unit(self) -> str: ...
+    def as_unit(self, unit: str, round_ok: bool = ...) -> Timestamp: ...