input: Write a function to remove special characters from a string.
  assert remove_special_characters("This# 1$ i^s an% e*xa_mple -str_ing") == "This 1 is an example -str ing"
  assert remove_special_characters("Hello$ W^or*ld!") == "Hello World!"
  assert remove_special_characters("S_p#e$c%i_a*l_ C*h_a^r%a*c$t%e$r_s") == "Special Characters"
output: def remove_special_characters(s):
      return ''.join(c for c in s if c.isalnum() or c.isspace())