Add numpydoc/tests/hooks to manifest

[jarrodmillman]

numpydoc/tests/hooks is in the source directory:

$ ls -1 numpydoc/tests/
hooks
test_docscrape.py
test_full.py
test_main.py
test_numpydoc.py
test_validate.py
test_xref.py
tinybuild

but isn't in the installed package

$ ls -1 ~/.venv/nx/lib64/python3.11/site-packages/numpydoc/tests/
test_docscrape.py
test_full.py
test_main.py
test_numpydoc.py
test_validate.py
test_xref.py
tinybuild

I noticed this in #555.