bpo-41306: Allow scale value to not be rounded (GH-21715)

bpo-41306: Allow scale value to not be rounded (GH-21715)

This fixes the test failure with Tk 6.8.10 which is caused by changes to how Tk rounds the `from`, `to` and `tickinterval` arguments. This PR uses `noconv` if the patchlevel is greater than or equal to 8.6.10 (credit to Serhiy for this idea as it is much simpler than what I previously proposed).

Going into more detail for those who want it, the Tk change was made in [commit 591f68c](https://github.com/tcltk/tk/commit/591f68cb382525b72664c6fecaab87742b6cc87a) and means that the arguments listed above are rounded relative to the value of `from`. However, when rounding the `from` argument ([line 623](https://github.com/tcltk/tk/blob/591f68cb382525b72664c6fecaab87742b6cc87a/generic/tkScale.c#L623)), it is rounded relative to itself (i.e. rounding `0`) and therefore the assigned value for `from` is always what is given (no matter what values of `from` and `resolution`).

Automerge-Triggered-By: @pablogsal