build-install-dumb-init: pip install py

Since pytest-dev/pytest#10396 the py library is
no longer installed, but dumb-init needs 'py._path'.

Fixes:

ImportError while importing test module '/tmp/tmp.k8zSDBKT6S/dumb-init-1.2.5/tests/child_processes_test.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
/usr/lib/python3.8/importlib/__init__.py:127: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
tests/child_processes_test.py:10: in <module>
    from testing import is_alive
testing/__init__.py:11: in <module>
    from py._path.local import LocalPath
E   ModuleNotFoundError: No module named 'py._path'; 'py' is not a package

Issue filed upstream:
Yelp/dumb-init#286

Signed-off-by: Tim Orling <tim.orling@konsulko.com>

build-install-dumb-init.sh

@@ -58,7 +58,7 @@ pip3 install virtualenv || exit 1
 
 virtualenv $builddir/venv || exit 1
 . $builddir/venv/bin/activate || exit 1
-pip3 install setuptools tox || exit 1
+pip3 install setuptools tox py || exit 1
 )
 
 . $builddir/venv/bin/activate || exit 1