Add encoding declaration

I was seeing the following error on Python 2.7:

    In [1]: import calendar as cal
      File "calendar.py", line 80
        return "La Fête de la Révolution de l'Année " + roman_year + " de la Revolution"
                                                                                        ^
    SyntaxError: Non-ASCII character '\xc3' in file calendar.py on line 80

Adding a declaration comment squashes that error.