data wiped from `MemoryStore` after `store.close` + `array[:]`

[d-v-b]

Over in #1746 @dcherian discovered the following bug:

import zarr
from zarr.array import Array
from zarr.group import Group
from zarr.store import MemoryStore
import numpy as np

store = MemoryStore(mode="w")
root = Group.create(store)
nparray = np.array([1], dtype=np.int8)
    
a = root.create_array(
    "/0/0",
    shape=nparray.shape,
    chunks=(1,),
    dtype=nparray.dtype.str,
    attributes={},
    # compressor=compressor,  # TODO: FIXME
    fill_value=nparray.dtype.type(0),
)
a[:] = nparray
print(a[:]) # [1]
store.close()
print(a[:]) # [0]

Setting data with a[:] = nparray, followed by a.store_path.store.close(), followed by a[:], results in the store getting wiped. This is not intended behavior.

this PR adds:

• type hints to the indexing tests
• a test for the aforementioned bug, which is currently failing, because we have not fixed it yet. this PR should be updated when we fix the bug.

TODO:

• [ ] Add unit tests and/or doctests in docstrings
• [ ] Add docstrings and API docs for any new/modified user-facing classes and functions
• [ ] New/modified features documented in docs/tutorial.rst
• [ ] Changes documented in docs/release.rst
• [ ] GitHub Actions have all passed
• [ ] Test coverage is 100% (Codecov passes)

Closes https://github.com/zarr-developers/zarr-python/pull/2067