default message for assert_isinstance

[Jasha10]

This PR implements a default message for assert_isinstance.

Consider the following example:

# tmp.py
from typing import Any

import numpy as np
from nptyping import NDArray, assert_isinstance

my_arr = np.array([1, 2, 3], dtype=np.float32)
assert_isinstance(my_arr, NDArray[Any, np.int64])

Before the PR:

$ python tmp.py
Traceback (most recent call last):
...
AssertionError

The above offers little info as to what caused the failure.

After the PR:

$ python tmp.py
Traceback (most recent call last):
...
AssertionError: instance=array([1., 2., 3.], dtype=float32), cls=NDArray[Any, Int]

This should be enough to diagnose why the assertion failed.

[sonarcloud[bot]]

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