Use yaml.safe_load() to fix deprecation and crash

The `yaml.load()` function without a `Loader` keyword is deprecated
since `pyyaml 5.1` and removed with `pyyaml 6.0`.

Ubuntu 24.04 noble ships with `pyyaml 6.0.1` breaking `python-aptly` and
`aptly-publisher`.

Fix it by using the recommended `yaml.safe_load()` function.

Fixes: tcpcloud#32

aptly/publisher/__init__.py

@@ -13,7 +13,7 @@
 
 def load_publish(publish):
     with open(publish, 'r') as publish_file:
-        return yaml.load(publish_file)
+        return yaml.safe_load(publish_file)
 
 
 class PublishManager(object):

aptly/publisher/__main__.py

@@ -20,7 +20,7 @@
 
 def load_config(config):
     with open(config, 'r') as fh:
-        return yaml.load(fh)
+        return yaml.safe_load(fh)
 
 
 def get_latest_snapshot(snapshots, name):