♻️(back) replace Ypy by pycrdt

Ypy is deprecated and unmaintained. We have problem with parsing
existing documents. We replace it by pycrdt, library actively maintained
and without the issues we have with Ypy.

src/backend/core/tests/migrations/test_migrations_0020_remove_is_public_add_field_attachments_and_duplicated_from.py

@@ -4,8 +4,8 @@ import uuid
 from django.core.files.base import ContentFile
 from django.core.files.storage import default_storage
 
+import pycrdt
 import pytest
-import y_py
 
 from core import models
 
@@ -27,14 +27,13 @@ def test_populate_attachments_on_all_documents(migrator):
 
     # Create document content with an image
     file_key = f"{old_doc_with_attachments.id!s}/file"
-    ydoc = y_py.YDoc()  # pylint: disable=no-member
     image_key = f"{old_doc_with_attachments.id!s}/attachments/{uuid.uuid4()!s}.png"
-    with ydoc.begin_transaction() as txn:
-        xml_fragment = ydoc.get_xml_element("document-store")
-        xml_fragment.push_xml_element(txn, "image").set_attribute(
-            txn, "src", f"http://localhost/media/{image_key:s}"
-        )
-    update = y_py.encode_state_as_update(ydoc)  # pylint: disable=no-member
+    ydoc = pycrdt.Doc()
+    fragment = pycrdt.XmlFragment(
+        [pycrdt.XmlElement("img", {"src": f"http://localhost/media/{image_key:s}"})]
+    )
+    ydoc["document-store"] = fragment
+    update = ydoc.get_update()
     base64_content = base64.b64encode(update).decode("utf-8")
     bytes_content = base64_content.encode("utf-8")
     content_file = ContentFile(bytes_content)