Downgrade pillow to 9.5.0

The latest blockdiag is not compatible with pillow 10.y.z[1].

The library looks unmaintained we may have to consider replacing it,
but in the mean time let's downgrade pillow to fix broken nova doc
builds.

[1] blockdiag/blockdiag#178

Change-Id: I58f0c06f122fd3b5ad349d1bdd25d5417dc0e39d

upper-constraints.txt

@@ -324,7 +324,7 @@ pydotplus===2.0.2
 boto3===1.34.23
 jeepney===0.8.0
 stestr===4.1.0
-pillow===10.2.0
+pillow===9.5.0
 pysmi-lextudio===1.1.13
 oslo.serialization===5.3.0
 warlock===2.0.1