Fix the following errors:
```
osbuild/util/lvm2.py:117: error: Only instance methods can be decorated with @property
osbuild/api.py:50: error: Only instance methods can be decorated with @property
osbuild/sources.py:85: error: Only instance methods can be decorated with @property
```
Chaining of `@classmethod` and `@property` has been deprecated since
Python 3.11 with a note that chaining didn't work correctly in some
cases.
Relevant links:
https://github.com/python/mypy/issues/13746https://docs.python.org/3.11/whatsnew/3.11.html#language-builtins
Signed-off-by: Tomáš Hozza <thozza@redhat.com>