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](tcltk/tk@591f68c) 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.cGH-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
(cherry picked from commit aecf036)

Co-authored-by: E-Paine <[email protected]>

Author: miss-islington

Lib/tkinter/test/test_tkinter/test_widgets.py

@@ -940,7 +940,8 @@ def test_digits(self):
 
     def test_from(self):
         widget = self.create()
-        self.checkFloatParam(widget, 'from', 100, 14.9, 15.1, conv=float_round)
+        conv = False if get_tk_patchlevel() >= (8, 6, 10) else float_round
+        self.checkFloatParam(widget, 'from', 100, 14.9, 15.1, conv=conv)
 
     def test_label(self):
         widget = self.create()

Misc/NEWS.d/next/Tests/2020-08-03-13-44-37.bpo-41306.VDoWXI.rst

@@ -0,0 +1 @@
+Fixed a failure in ``test_tk.test_widgets.ScaleTest`` happening when executing the test with Tk 8.6.10.