slack: Requote image_url in render_attachment.

Slack attachment urls with white spaces,
e.g. `https://example.com/some file.png`,
were rejected by `check_url`. We want to call `requote_url` to deal with
any url-quoting jankiness that may be present in the exported data.

zerver/data_import/slack_message_conversion.py

@@ -5,6 +5,7 @@ from typing import Any, Literal, TypeAlias, TypedDict, cast
 
 import regex
 from django.core.exceptions import ValidationError
+from requests.utils import requote_uri
 
 from zerver.lib.types import Validator
 from zerver.lib.validator import (
@@ -423,8 +424,13 @@ def render_attachment(attachment: WildValue) -> str:
         pieces.append("\n".join(fields))
     if attachment.get("blocks"):
         pieces += map(render_block, attachment["blocks"])
-    if attachment.get("image_url"):
-        pieces.append("[]({})".format(attachment["image_url"].tame(check_url)))
+    if image_url_wv := attachment.get("image_url"):
+        try:
+            image_url = image_url_wv.tame(check_url)
+        except ValidationError:  # nocoverage
+            image_url = image_url_wv.tame(check_string)
+            image_url = requote_uri(image_url)
+        pieces.append(f"[]({image_url})")
     if attachment.get("footer"):
         pieces.append(attachment["footer"].tame(check_string))
     if attachment.get("ts"):