stack/slapos.cfg: Change cattrs version to wheel

Now that buildout uses pip to correctly follow pyproject.toml,
cattrs=22.2.0 requires poetry as the build backend.

For now it's easier to install as wheel than to install poetry and
then use it as setup-egg for cattrs. This must be done eventually.

stack/slapos.cfg

@@ -165,7 +165,7 @@ bleach = 5.0.1
 blinker = 1.6.3:whl
 CacheControl = 0.12.6:whl
 cachetools = 5.3.1
-cattrs = 22.2.0
+cattrs = 22.2.0:whl
 certifi = 2023.7.22
 cffi = 1.15.0
 chardet = 3.0.4