External references ignore memmap argument passed to open

[braingram]

Example of the problem

import asdf
import numpy as np

arr = np.arange(42)
asdf.AsdfFile({"arr": arr}).write_to("external.asdf")

tree = {
    "rarr": {"$ref": "external.asdf#/arr"},
}

asdf.AsdfFile(tree).write_to("foo.asdf")

with asdf.open("foo.asdf", memmap=False) as af:
    af.find_references()
    af["rarr"][0] = 1

results in

ValueError: assignment destination is read-only

even though memmap=False was passed to asdf.open (note that this example will start to pass with asdf 4.0.0 but only because the default for memmap will be set to False, the example could be updated to instead check for memmap=True and a ValueError where it will fail for asdf 4.0.0)

asdf version: main
python version: 3.10
operating system: mac