Describe the bug
Method as_unit not defined on Timestamp

To Reproduce

1. Provide a minimal runnable pandas example that is not properly checked by the stubs.
   pd.Timestamp(2023, 1, 1).as_unit("ns")
2. Indicate which type checker you are using (mypy or pyright).
   mypy
3. Show the error message received from that type checker while checking your example.
   error: "Timestamp" has no attribute "as_unit" [attr-defined]

Please complete the following information:

• OS: Linux
• OS Fedora 7
• python version: 3.11
• version of type checker: 1.8
• version of installed pandas-stubs=2.1.4