input: Write a python function to connect to a PostgreSQL database, execute a query, and fetch all results.
  assert fetch_results_from_db("SELECT * FROM table_name") == [(1, 'Alice'), (2, 'Bob')]
  assert fetch_results_from_db("SELECT * FROM table_name WHERE id=1") == [(1, 'Alice')]
  assert fetch_results_from_db("SELECT * FROM empty_table") == []
output: def fetch_results_from_db(query):
      import psycopg2
      conn = psycopg2.connect(
          dbname="your_dbname",
          user="your_username",
          password="your_password",
          host="your_host",
          port="your_port"
      )
      cur = conn.cursor()
      cur.execute(query)
      results = cur.fetchall()
      cur.close()
      conn.close()
      return results