Improve Python 3.9 compatibility

Blist has been unmaintained for 5+ years and is now broken on Python 3.9.

packaging/flatpak/mirage.flatpak.yaml

@@ -233,15 +233,6 @@ modules:
   - type: file
     url: https://files.pythonhosted.org/packages/66/25/ff030e2437265616a1e9b25ccc864e0371a0bc3adb7c5a404fd661c6f4f6/beautifulsoup4-4.9.1-py3-none-any.whl
     sha256: a6237df3c32ccfaee4fd201c8f5f9d9df619b93121d01353a64a73ce8c6ef9a8
-- name: python3-blist
-  buildsystem: simple
-  build-commands:
-  - pip3 install --exists-action=i --no-index --find-links="file://${PWD}" --prefix=${FLATPAK_DEST}
-    "blist==1.3.6"
-  sources:
-  - type: file
-    url: https://files.pythonhosted.org/packages/6b/a8/dca5224abe81ccf8db81f8a2ca3d63e7a5fa7a86adc198d4e268c67ce884/blist-1.3.6.tar.gz
-    sha256: 3a12c450b001bdf895b30ae818d4d6d3f1552096b8c995f0fe0c74bef04d1fc3
 - name: python3-cachetools
   buildsystem: simple
   build-commands:

requirements.txt

@@ -1,7 +1,6 @@
 Pillow          >= 7.0.0,  < 8
 aiofiles        >= 0.4.0,  < 0.5
 appdirs         >= 1.4.4,  < 2
-blist           >= 1.3.6,  < 2
 cairosvg        >= 2.4.2,  < 3
 filetype        >= 1.0.7,  < 2
 html_sanitizer  >= 1.9.1,  < 2

src/backend/models/model.py

@@ -9,8 +9,6 @@ from typing import (
     TYPE_CHECKING, Any, Dict, Iterator, List, MutableMapping, Optional, Tuple,
 )
 
-from blist import blist
-
 from ..pyotherside_events import ModelCleared, ModelItemDeleted, ModelItemSet
 from . import SyncId
 
@@ -41,7 +39,7 @@ class Model(MutableMapping):
         self.sync_id:      Optional[SyncId]       = sync_id
         self.write_lock:   RLock                  = RLock()
         self._data:        Dict[Any, "ModelItem"] = {}
-        self._sorted_data: List["ModelItem"]      = blist()
+        self._sorted_data: List["ModelItem"]      = []
 
         self.take_items_ownership: bool = True