chore: Replace deprecated smart_text with smart_str

Part-of: <https://dev.funkwhale.audio/funkwhale/funkwhale/-/merge_requests/2709>

api/funkwhale_api/common/serializers.py

@@ -5,7 +5,7 @@ import os
 import PIL
 from django.core.exceptions import ObjectDoesNotExist
 from django.core.files.uploadedfile import SimpleUploadedFile
-from django.utils.encoding import smart_text
+from django.utils.encoding import smart_str
 from django.utils.translation import gettext_lazy as _
 from drf_spectacular.types import OpenApiTypes
 from drf_spectacular.utils import extend_schema_field
@@ -52,7 +52,7 @@ class RelatedField(serializers.RelatedField):
             self.fail(
                 "does_not_exist",
                 related_field_name=self.related_field_name,
-                value=smart_text(data),
+                value=smart_str(data),
             )
         except (TypeError, ValueError):
             self.fail("invalid")