chore: update python migration template format to use triple quotes for (#356)

multiline statement

The Python default migration template uses double quotes for the upgrade
statement, which is multiline and therefore invalid syntax.
This replaces the start and end quotes with triple quotes instead.

Co-authored-by: Jordan Russell <jordan.russell@sitehost.co.nz>

Author: ftsartek

sqlspec/migrations/templates.py

@@ -221,12 +221,12 @@ def _normalize_newlines(lines: "list[str]") -> "list[str]":
     body=(
         "def up(context: object | None = None) -> str | Iterable[str]:\n"
         '    """Apply the migration (upgrade)."""\n'
-        '    return "\n'
+        '    return """\n'
         "    CREATE TABLE example (\n"
         "        id INTEGER PRIMARY KEY,\n"
         "        name TEXT NOT NULL\n"
         "    );\n"
-        '    "\n\n'
+        '    """\n\n'
         "def down(context: object | None = None) -> str | Iterable[str]:\n"
         '    """Reverse the migration."""\n'
         '    return "DROP TABLE example;"'