fix: Remove deprecated ___ separator support

Only --- is now accepted as the primary key separator.
This simplifies the syntax and aligns with documented usage.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: dimitri-yatsenko

src/datajoint/declare.py

@@ -360,7 +360,7 @@ def prepare_declare(
     for line in definition:
         if not line or line.startswith("#"):  # ignore additional comments
             pass
-        elif line.startswith("---") or line.startswith("___"):
+        elif line.startswith("---"):
             in_key = False  # start parsing dependent attributes
         elif is_foreign_key(line):
             compile_foreign_key(