nbconvert removed .tests submodule from published packages

[bnavigator]

This results in the following error when running the test suite (with pytest, see #1587, but that should be irrelevant here)

[   79s] ============================= test session starts ==============================
[   79s] platform linux -- Python 3.8.14, pytest-7.1.2, pluggy-1.0.0 -- /usr/bin/python3.8
[   79s] cachedir: .pytest_cache
[   79s] rootdir: /home/abuild/rpmbuild/BUILD/jupyter_contrib_nbextensions-0.5.1
[   83s] collecting ... collected 23 items / 1 error / 1 deselected / 22 selected
[   83s] 
[   83s] ==================================== ERRORS ====================================
[   83s] ___________________ ERROR collecting tests/test_exporters.py ___________________
[   83s] ImportError while importing test module '/home/abuild/rpmbuild/BUILD/jupyter_contrib_nbextensions-0.5.1/tests/test_exporters.py'.
[   83s] Hint: make sure your test modules/packages have valid Python names.
[   83s] Traceback:
[   83s] /usr/lib64/python3.8/importlib/__init__.py:127: in import_module
[   83s]     return _bootstrap._gcd_import(name[level:], package, level)
[   83s] tests/test_exporters.py:9: in <module>
[   83s]     from nbconvert.tests.base import TestsBase
[   83s] E   ModuleNotFoundError: No module named 'nbconvert.tests'

[GaetanLepage]

I was able to replicate the same behavior.