Fix __str__ for EventFiniteNominal to quote the actual nominal values

[fsaad]

X << ['a', 'b']

should render as

X << {'a', 'b'}

so as to distinguish between symbolic entities (x) and string entities ('a', 'b') in the language.

Similarly, for negation we have

~(X << ['a', 'b'])

should render as

X << UniversalSet() \ {'a', 'b'}

[fsaad]

I remember now that sympy.FiniteSet('0') will automatically convert the string '0' to an integer 0, so NominalValue(Atom) tricks sympy into thinking that '0' cannot be converted (by overriding its 'str' method).

[fsaad]

Sympy now plan to remove this horrible feature :)
sympy/sympy#18066

[fsaad]

Fixed by removing the Sympy dependence #93