error in Time transform of 9999-12-31

## Issue Description
i want to change '9999-12-31' to timestamp, and also in anather scene, i need change the timestamp(253402214400) to datetime.
but I got confused when I test this :
```
dt2=arrow.get('9999-12-31')
dt2.timestamp()
```
then get:
> 253402214400.0

and change it to datetime:
`arrow.get(253402214400.0)`

but get :
>

this code should get result like of : "9999-12-31 "

So, It is a bug , or my code is not correct！

## System Info

- 🖥 **OS name and version**: winodws11 and wsl ubuntu
- 🐍 **Python version**: python3.7
- 🏹 **Arrow version**: 1.2.3

Reproduce the two Arrow examples with Python 3.7 and Arrow 1.2.3, then trace how timestamp values are interpreted when passed to arrow.get. Check the relevant date conversion tests or add a focused round-trip case; done means converting the reported date to a timestamp and back produces the expected date or clearly documents the supported units.

Written by the indexing model from the issue text.