Fix [p]cleanup self not working in DMs for non-owners (#4481)

* Fix cleanup self not working in private.

This fix #4408.

* Apply Jack's logic.

* Obviously Black fault.
AGAIN!

* Apply Jack's trick.

* This is not a converter so let's move this to a different file

Co-authored-by: jack1142 <6032823+jack1142@users.noreply.github.com>

redbot/cogs/cleanup/checks.py

@@ -0,0 +1,15 @@
+from redbot.core.commands import Context, permissions_check
+from redbot.core.utils.mod import is_mod_or_superior, check_permissions
+
+
+def check_self_permissions():
+    async def predicate(ctx: Context):
+        if not ctx.guild:
+            return True
+        if await check_permissions(ctx, {"manage_messages": True}) or await is_mod_or_superior(
+            ctx.bot, ctx.author
+        ):
+            return True
+        return False
+
+    return permissions_check(predicate)