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

osbuild/sources.py

@@ -4,8 +4,7 @@ import contextlib
 import json
 import os
 import tempfile
-from abc import abstractmethod
-from typing import Dict, Tuple
+from typing import ClassVar, Dict, Tuple
 
 from . import host
 from .objectstore import ObjectStore
@@ -55,6 +54,9 @@ class SourceService(host.Service):
 
     max_workers = 1
 
+    content_type: ClassVar[str]
+    """The content type of the source."""
+
     def __init__(self, *args, **kwargs):
         super().__init__(*args, **kwargs)
         self.cache = None
@@ -82,12 +84,6 @@ class SourceService(host.Service):
             for _ in executor.map(self.fetch_one, *zip(*transformed)):
                 pass
 
-    @property
-    @classmethod
-    @abstractmethod
-    def content_type(cls):
-        """The content type of the source."""
-
     @staticmethod
     def load_items(fds):
         with os.fdopen(fds.steal(0)) as f:

osbuild/util/lvm2.py

@@ -18,7 +18,6 @@ LVM2 sources[1], specifically:
 [1] https://github.com/lvmteam/lvm2 (commit 8801a86)
 """
 
-import abc
 import binascii
 import io
 import json
@@ -27,7 +26,7 @@ import re
 import struct
 import sys
 from collections import OrderedDict
-from typing import BinaryIO, Dict, List, Union
+from typing import BinaryIO, ClassVar, Dict, List, Union
 
 PathLike = Union[str, bytes, os.PathLike]
 
@@ -114,10 +113,7 @@ class CStruct:
 class Header:
     """Abstract base class for all headers"""
 
-    @property
-    @classmethod
-    @abc.abstractmethod
-    def struct(cls) -> Union[struct.Struct, CStruct]:
+    struct: ClassVar[Union[struct.Struct, CStruct]]
     """Definition of the underlying struct data"""
 
     def __init__(self, data):