Fix errors reported by new version of mypy

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/13746
https://docs.python.org/3.11/whatsnew/3.11.html#language-builtins

Signed-off-by: Tomáš Hozza <thozza@redhat.com>

osbuild/api.py

@@ -8,7 +8,7 @@ import sys
 import tempfile
 import threading
 import traceback
-from typing import Dict, Optional
+from typing import ClassVar, Dict, Optional
 
 from .util import jsoncomm
 from .util.types import PathLike
@@ -40,6 +40,9 @@ class BaseAPI(abc.ABC):
     call `_message.`
     """
 
+    endpoint: ClassVar[str]
+    """The name of the API endpoint"""
+
     def __init__(self, socket_address: Optional[PathLike] = None):
         self.socket_address = socket_address
         self.barrier = threading.Barrier(2)
@@ -47,12 +50,6 @@ class BaseAPI(abc.ABC):
         self.thread = None
         self._socketdir = None
 
-    @property
-    @classmethod
-    @abc.abstractmethod
-    def endpoint(cls):
-        """The name of the API endpoint"""
-
     @abc.abstractmethod
     def _message(self, msg: Dict, fds: jsoncomm.FdSet, sock: jsoncomm.Socket):
         """Called for a new incoming message