FIX: sympify unevaluated_expression instance attributes

[redeboer]

Classes with attributes that are not of type sp.Basic would previously crash. For instance:

import sympy as sp
from ampform.sympy import unevaluated_expression
from typing import Any

@unevaluated_expression
class MyExpr(sp.Expr):
    x: Any
    m: Any

    def evaluate(self) -> sp.Expr:
        return self.x**self.m

expr = MyExpr(4, m=2)
assert expr.doit() == 16

would previously crash, because the value given to x is of type int and was not sympified to sp.Integer.