CI: update minimum versions following SPEC0 as of Q4 2023

Author: bsipocz

tox.ini

@@ -15,10 +15,10 @@ deps =
     -rsite/requirements.txt
     -rrequirements.txt
 
-    oldestdeps: numpy==1.20
-    oldestdeps: matplotlib==3.4
-    oldestdeps: scipy==1.6
-    oldestdeps: pandas==1.2
+    oldestdeps: numpy==1.23
+    oldestdeps: matplotlib==3.6
+    oldestdeps: scipy==1.8
+    oldestdeps: pandas==1.4
 
 allowlist_externals = bash