format XSSS outputs too many digits and a dot

## Issue Description

New release changed the behaviour of `.format('XSSS')`, for example:

on 0.17.0 `arrow.utcnow().format('XSSS')` yields `'1614555372217'`

on 1.0.2 it yields `'1614556907.369226369'`

## System Info

- 🖥 **OS name and version**: macOS 10.15.7 (visible also on ubutnu and windows)
- 🐍 **Python version**: 3.7.10
- 🏹 **Arrow version**: 1.0.2

Start at the `.format('XSSS')` entry point and reproduce the reported outputs on the affected versions. Trace how this token is formatted, then verify that the result uses the intended millisecond value without a decimal point or excess digits.

Written by the indexing model from the issue text.