Fix a bug in pyfaust.Faust.__getitem__().

Example:
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ERROR: testGetItem (__main__.TestFaustPy.testGetItem)
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Traceback (most recent call last):
  File /home/gitlab-runner/builds/YstEs9tG/0/faustgrp/faust/misc/test/src/Python//test_FaustPy.py, line 376, in testGetItem
    norm(F_rows.toarray()-F.toarray()[row_ids,col_slice_start:col_slice_stop])
         ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
ValueError: operands could not be broadcast together with shapes (28,23) (28,3)

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https://faustgrp.gitlabpages.inria.fr/-/faust/-/jobs/3275988/artifacts/build_FaustLinuxPython/python_pyfaust_test_output.html#ace4ae8e-3b29-4f14-aca6-20806b4c197d