bigarray: tests: Do not use `numpy.object`.

`numpy.object` was an alias for the builtin `object`, so we can use
`object` instead:

```python
_________________________ test_bigarray_noobject[tRAM] _________________________

testbig = <bigarray.tests.test_basic.tRAM object at 0x7f6d114ead60>

    def test_bigarray_noobject(testbig):
        Zh = testbig.fopen()

        # NOTE str & unicode are fixed-size types - if size is not explicitly given
        # it will become S0 or U0
>       obj_dtypev = [numpy.object, 'O', 'i4, O', [('x', 'i4'), ('y', 'i4, O')]]

bigarray/tests/test_basic.py:110:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

attr = 'object'

    def __getattr__(attr):
        # Warn for expired attributes, and return a dummy function
        # that always raises an exception.
        import warnings
        import math
        try:
            msg = __expired_functions__[attr]
        except KeyError:
            pass
        else:
            warnings.warn(msg, DeprecationWarning, stacklevel=2)

            def _expired(*args, **kwds):
                raise RuntimeError(msg)

            return _expired

        # Emit warnings for deprecated attributes
        try:
            val, msg = __deprecated_attrs__[attr]
        except KeyError:
            pass
        else:
            warnings.warn(msg, DeprecationWarning, stacklevel=2)
            return val

        if attr in __future_scalars__:
            # And future warnings for those that will change, but also give
            # the AttributeError
            warnings.warn(
                f"In the future `np.{attr}` will be defined as the "
                "corresponding NumPy scalar.", FutureWarning, stacklevel=2)

        if attr in __former_attrs__:
>           raise AttributeError(__former_attrs__[attr])
E           AttributeError: module 'numpy' has no attribute 'object'.
E           `np.object` was a deprecated alias for the builtin `object`. To avoid this error in existing code, use `object` by itself. Doing this will not modify any behavior and is safe.
E           The aliases was originally deprecated in NumPy 1.20; for more details and guidance see the original release note at:
E               https://numpy.org/devdocs/release/1.20.0-notes.html#deprecations
```

--------
kirr:

On py2:

    In [1]: import numpy

    In [2]: numpy.__version__
    Out[2]: '1.16.6'

    In [3]: numpy.object
    Out[3]: object

    In [4]: numpy.object is object
    Out[4]: True

this change is, thus, indeed safe to make.

/reviewed-by @kirr
/reviewed-on !27

bigarray/tests/test_basic.py

@@ -107,7 +107,7 @@ def test_bigarray_noobject(testbig):
 
     # NOTE str & unicode are fixed-size types - if size is not explicitly given
     # it will become S0 or U0
-    obj_dtypev = [numpy.object, 'O', 'i4, O', [('x', 'i4'), ('y', 'i4, O')]]
+    obj_dtypev = [object, 'O', 'i4, O', [('x', 'i4'), ('y', 'i4, O')]]
     for dtype_ in obj_dtypev:
         with raises(TypeError):
             BigArray((1,), dtype_, Zh)