Draft: Fix annotation scale to label issue.

[Roy-043]

The scale_to_label function did not handle scale 5:1, 10:1 or 20:1 properly.

Forum post: https://forum.freecadweb.org/viewtopic.php?f=23&t=60398

The cause of the problem:

>>> (1/20).as_integer_ratio()
(3602879701896397, 72057594037927936)
>>> (1/10).as_integer_ratio()
(3602879701896397, 36028797018963968)
>>> (1/5).as_integer_ratio()
(3602879701896397, 18014398509481984)

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[chennes]

Good catch, thanks for the PR. Merging...