Add `testing` to `__all__` in `src/array_api_extra/__init__.py`

[paddyroddy]

Currently, we can't do

import array_api_extra as xpx

xpx.testing.assert_close(...)

[prady0t]

If we just want to import via xpx, we don't need to add testing to __all__. We can just do

from . import testing

in the __init__.py file.
Happy to add that, as that would also align with np.testing import behaviour. But not sure about adding it to __all__, as that would imply we can also import it as:

from array_api_extra import *

Which is unnecessary for the testing module.

[paddyroddy]

If we just want to import via xpx, we don't need to add testing to __all__. We can just do

from . import testing

in the __init__.py file. Happy to add that, as that would also align with np.testing import behaviour.

Ah yeah, my bad. This is a much better idea!

[lucascolley]

On the other hand, SciPy does include submodules in __all__... https://github.com/scipy/scipy/blob/main/scipy/__init__.py#L95-L120

[prady0t]

Actually, numpy imports submodules too:

https://github.com/numpy/numpy/blob/dd4339c40d375902306dea1ee6d834ac5f75ed6c/numpy/__init__.py#L675
https://github.com/numpy/numpy/blob/dd4339c40d375902306dea1ee6d834ac5f75ed6c/numpy/__init__.py#L628
I guess we can add testing to __all__ then.