Make SymbolicDFA accept SymPy symbols as keys of propositional interpretations

[marcofavorito]

Is your feature request related to a problem? Please describe.
The following code does not work:

from pythomata.impl.symbolic import SymbolicDFA
from sympy import symbols
a = symbols("a")
dfa = SymbolicDFA()
dfa.create_state()
dfa.add_transition((0, a, 1))
dfa.set_accepting_state(1, True)

# returns True, correctly
dfa.accepts([{"a": True}])

# it raises ValueError: Symbol {a: True} is not valid.
dfa.accepts([{a: True}])

The reason is that SymbolicDFA.accepts accepts only PropositionalInterpretation whose labels are of type str and not sympy.Symbol.

Describe the solution you'd like
Make it work. That is, try to be more flexible on the data validation in SymbolicDFA.accepts.

Describe alternatives you've considered
n/a

Additional context
n/a