import bson

filename = "C:\\Users\\Admin\\Documents\\Test
test.bson"
bs = open(filename, 'rb').read()
data = bson.decode_all(bs)

leads to output:

data = bson.decode_all(bs)
<ipython-input-14-46e487bf5e6b>:1: DeprecationWarning: PY_SSIZE_T_CLEAN will be required for '#' formats
data = bson.decode_all(bs)

The same warning is mentioned in other packages, it seems to be a recent deprecation, for example:

https://github.com/rdiff-backup/rdiff-backup/issues/374

via https://github.com/zopefoundation/persistent/issues/108