Implement 'selector-append' sass function

'selector-append' is used in xstatic-mdi [1] but it isn't supported by
the latest pyScss.
As a workaround we use 'append-selector' function from pyScss until
'selector-append' will be supported.

This patch also reverts Ia9e1e807591d4428f585177f521d4cb9d463b917
because we don't need it anymore.

[1] https://opendev.org/openstack/xstatic-mdi/src/tag/1.6.50.2/xstatic/pkg/mdi/data/scss/_functions.scss#L2

stable/train backport:
A release note on the known issue is now removed as it is fixed
by this commit. Instead, a note on the workaround by this commit
is added.

Change-Id: Ib8ca0fcfe339cb68d6a157a00cbc2d34854ef4c5
Closes-Bug: #1771559
(cherry picked from commit 74d76e900f)

openstack_dashboard/themes/material/static/horizon/_styles.scss

@@ -1,3 +1,12 @@
+// NOTE(e0ne): it's temporary workaround to until specified function will
+// be supported by pyScss. We need to define this function before any MDI
+// usage.
+@if not function-exists("selector-append") {
+  @function selector-append($selector, $to-append) {
+    @return append-selector($selector, $to-append);
+  }
+}
+
 @import "animations";
 @import "icons";
 @import "components/checkboxes";

releasenotes/notes/pyscss-and-setuptools-7634dcee9d62bde9.yaml

@@ -1,13 +1,9 @@
 ---
-issues:
-  - |
-    The material design theme is now broken with pyScss 1.3.7
-    (also see the upgrade notice section).
-    If you really depends on the material design theme, keep using
-    pyScss 1.3.4 with setuptools<45 (while the horizon dependencies list
-    says pyScss>=1.3.7 is required).
-    The issue is tracked via [:bug:`1771559`].
 upgrade:
   - |
     pyScss 1.3.7 is now required due to the incompatibility that
     pyScss 1.3.4 used before in horizon does not work with setuptools 46.
+  - |
+    Along with the upgrade of pyScss to 1.3.7, a workaround for the
+    material design theme is added to make it work properly.
+    See [:bug:`1771559`] for detail.