Fix over-precise float formatting on Python 3

[mgedmin]

Sometimes tox would tell me things like

✔ OK py27 in 4.1370000000000005 seconds
✔ OK py36 in 14.5 seconds
✔ OK py37 in 15.029 seconds
✔ OK py35 in 17.519 seconds
✔ OK py34 in 18.85 seconds

the almost-rounded-to-3-decimals value was both funny and sad, but mostly irritating.

This was happening because on Python 3 float.__str__ was changed to be more precise, and now

>>> print(4 + 0.137)
4.1370000000000005

We can avoid the problem if we round the result of the addition instead of adding a rounded number to the integer number of seconds:

>>> print(4.137)
4.137

I've also corrected the plural logic because "0.5 second" is wrong:
https://ell.stackexchange.com/questions/7817/singular-or-plural-for-seconds

Contribution checklist:

(also see CONTRIBUTING.rst for details)

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• updated/extended the documentation -- not applicable, I think
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