Remove python 3.9 support

OpenStack recently dropped python3.9 support for global requirements[1]
as it was removed from the Flamingo supported runtimes[2].

So this patch removes 3.9 support from nova too.

[1]https://review.opendev.org/c/openstack/requirements/+/948285
[2]https://governance.openstack.org/tc/reference/runtimes/2025.2.html#python

Change-Id: I8aea971d7972959c32d5175926cbaddb21839f8e

tox.ini

@@ -53,14 +53,14 @@ commands =
   env TEST_OSPROFILER=1 stestr run --combine --no-discover 'nova.tests.unit.test_profiler'
   stestr slowest
 
-[testenv:{unit,py3,py39,py310,py311,py312}]
+[testenv:{unit,py3,py310,py311,py312}]
 setenv =
   {[testenv]setenv}
 # we do not have any greenlet leaks in unit tests so enforce that
 # by making greenlet leaks a failure.
   NOVA_RAISE_ON_GREENLET_LEAK=True
 
-[testenv:functional{,-py39,-py310,-py311,-py312}]
+[testenv:functional{,-py310,-py311,-py312}]
 description =
   Run functional tests.
 setenv =