format XSSS outputs too many digits and a dot

[mpnowacki-reef]

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

[systemcatch]

Hi @mpnowacki-reef I don't think this is bug however it seems we did forget to mention the changes to the "X" token in the migration guide.

In 1.0.2.

>>> import arrow
>>> dt=arrow.utcnow()
>>> dt.format("X")
'1614632655.629982'

To get a microsecond timestamp.

>>> dt.format("x")
'1614632655629982'

However we may need to review this decision.