Prevent an implementation for `int` class from operating on `bool` values

[anatoly-scherbakov]

Context. Consider the following piece of code.

from classes import typeclass

@typeclass
def render(data_value) -> str:
    """Pretty-print a value."""

@render.instance(int)
def _render_int(data_value: int) -> str:
    return f'🔢 {data_value}'

render(True) == '🔢 True'

(this should be runnable as-is on classes 0.4.0.)

I would have expected this code to fail with a NotImplementedError because the bool case wasn't specified; but instead, that case is handled by the int implementation because

In [3]: issubclass(bool, int)
Out[3]: True

Decision. In an int implementation, recognize if the provided value is actually a bool and refuse to process that value.

Consequences. I have been writing in Python for quite a few years now and I might even have encountered this relationship between int and bool types before, but that is not quite a type of thing that I keep in my short-term memory to be able to instantly recognize.

I believe this change can help avert bugs which might be hard to trace otherwise.

[sobolevn]

Related python/mypy#8363