OverflowError: timestamp out of range for platform time_t

Found on i586 and 32-bit armv7l platforms in Radicale, any idea how to circumvent this?

```
[   29s]     dt = datetime.datetime.fromtimestamp(unixtime, datetime.UTC)
[   29s] OverflowError: timestamp out of range for platform time_t
```

See also: https://github.com/Kozea/Radicale/issues/1851