Remove reference to 3.7 and 3.8 backports. (GH-20754)

Stefan Krah · web-flow · commit 323188360d61 · 2020-06-09T10:27:45.000+02:00
diff --git a/Doc/library/decimal.rst b/Doc/library/decimal.rst
@@ -2193,4 +2193,3 @@ are expected to be exact.
 .. [#]
     .. versionchanged:: 3.9
        This approach now works for all exact results except for non-integer powers.
-       Also backported to 3.7 and 3.8.
