Fix eval of constants

[tomas789]

This fixes two problems.

First is that the classes _TRUE and _FALSE don't accept keyword arguments for the __call__ function. This means that when evaluating model with constants it raises a similar exception to this:

Traceback (most recent call last):
  File "/Users/tomaskrejci/Developer/quine-mccluskey/test.py", line 10, in <module>
    print(formula(**model))
          ^^^^^^^^^^^^^^^^
  File "/Users/tomaskrejci/Developer/quine-mccluskey/boolean.py/boolean/boolean.py", line 1596, in __call__
    return reduce(self._pyoperator, (a(**kwargs) for a in self.args))
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/tomaskrejci/Developer/quine-mccluskey/boolean.py/boolean/boolean.py", line 1596, in <genexpr>
    return reduce(self._pyoperator, (a(**kwargs) for a in self.args))
                                     ^^^^^^^^^^^
TypeError: _FALSE.__call__() got an unexpected keyword argument 'x0'

After adding the keyword arguments the second problem is that the __call__ magic function returns self instead of the boolean value. This leads to exception similar to this:

Traceback (most recent call last):
  File "/Users/tomaskrejci/Developer/quine-mccluskey/fuzzer.py", line 58, in <module>
    print(k, model, ones_formula(**model))
                    ^^^^^^^^^^^^^^^^^^^^^
  File "/Users/tomaskrejci/Developer/quine-mccluskey/boolean.py/boolean/boolean.py", line 1213, in __call__
    return not self.args[0](**kwargs)
               ^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/tomaskrejci/Developer/quine-mccluskey/boolean.py/boolean/boolean.py", line 1596, in __call__
    return reduce(self._pyoperator, (a(**kwargs) for a in self.args))
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/tomaskrejci/Developer/quine-mccluskey/boolean.py/boolean/boolean.py", line 881, in __or__
    return self.OR(self, other)
           ^^^^^^^^^^^^^^^^^^^^
  File "/Users/tomaskrejci/Developer/quine-mccluskey/boolean.py/boolean/boolean.py", line 1647, in __init__
    super(OR, self).__init__(arg1, arg2, *args)
  File "/Users/tomaskrejci/Developer/quine-mccluskey/boolean.py/boolean/boolean.py", line 1245, in __init__
    super(DualBase, self).__init__(arg1, arg2, *args)
  File "/Users/tomaskrejci/Developer/quine-mccluskey/boolean.py/boolean/boolean.py", line 1051, in __init__
    assert all(
AssertionError: Bad arguments: all arguments must be an Expression: (TRUE, False)

Code used to reproduce both bugs:

import boolean

algebra = boolean.BooleanAlgebra()
TRUE, FALSE, NOT, AND, OR, symbol = algebra.definition()

formula = FALSE
model = {"x0": False}

# This works
print(formula())

# This does not
print(formula(**model))