tox: Fix functional tests

Without this comma, tox runs the standard testenv instead of this one.
This is due to how tox calculates factors (parts of a testenv name) and
checks for their existence [1].

[1] tox-dev/tox#3219

Change-Id: I3fc33a26b0d7f057d7bd6a094fc2ceeaa88f177b
Signed-off-by: Stephen Finucane <[email protected]>

Author: stephenfin

tox.ini

@@ -23,7 +23,7 @@ commands =
     stestr run {posargs}
     stestr slowest
 
-[testenv:functional{-py39,-py310,-py311,-py312}]
+[testenv:functional{,-py39,-py310,-py311,-py312}]
 description =
     Run functional tests.
 # Some jobs (especially heat) takes longer, therefore increase default timeout