[3.1.x] Fixed #32391 -- Used CSS flex properties for changelist filter.

Matched layout adjustment using flex from admin sidebar added in
d24ba1be7a.

Filters would become squashed when viewport was
constrained or list display table became too wide.

Backport of 269a767146 from master

django/contrib/admin/static/admin/css/changelists.css

@@ -128,6 +128,7 @@
 /* FILTER COLUMN */
 
 #changelist-filter {
+    flex: 0 0 240px;
     order: 1;
     width: 240px;
     background: #f8f8f8;

django/contrib/admin/static/admin/css/responsive.css

@@ -157,7 +157,7 @@ input[type="submit"], button {
     }
 
     #changelist-filter {
-        width: 200px;
+        flex-basis: 200px;
     }
 
     .change-list .filtered .results,

docs/releases/3.1.6.txt

@@ -9,4 +9,5 @@ Django 3.1.6 fixes several bugs in 3.1.5.
 Bugfixes
 ========
 
-* ...
+* Fixed an admin layout issue in Django 3.1 where changelist filter controls 
+  would become squashed (:ticket:`32391`).