DiskBackedContent: add a small temporary compatibility layer

There are two other package using DiskBackedContent "swh-loader-svn" and
"swh-loader-cvs". Both use it to check "DiskBackedContent.object_type"
at the same time as "model.Content.object_type".

so we do this small hack to avoid breaking these other module until
they migrate.

requirements-test.txt

@@ -5,3 +5,4 @@ pytz
 types-click
 types-python-dateutil
 types-pytz
+types-deprecated

swh/model/from_disk.py

@@ -32,6 +32,7 @@ from typing import (
 import warnings
 
 import attr
+from deprecated import deprecated
 from typing_extensions import Final
 
 from . import model
@@ -65,6 +66,16 @@ class FromDiskType(model._StringCompatibleEnum):
         return hash(str(self.value))
 
 
+# There is an handful of other module that test for
+# DiskBackedContent.object_type in conjunction of Content.object_type, give
+# them a hand to let them handle compatibility in a smoother way.
+#
+# Remove this compatibility trick once this user have been migrated
+DiskBackedContent = deprecated(
+    version="v6.13.0", reason="Use model.Content.object_type instead"
+)(model.Content)
+
+
 @attr.s(frozen=True, slots=True)
 class DiskBackedData:
     path = attr.ib(type=bytes)