List all season numbers with production codes of 5m21.
CREATE TABLE "table1_23287683_1" (
  "series_num" real,
  "season_num" real,
  "title" text,
  "directed_by" text,
  "written_by" text,
  "original_air_date" text,
  "production_code" text
);
SELECT "season_num" FROM "table1_23287683_1" WHERE "production_code"='5M21';