build-install-dumb-init: install py

Append 'py' to dumb-init*/requirements-dev.txt

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

@@ -38,6 +38,8 @@ sha256sum -c sha256sums || exit 1
 tar xf v1.2.5.tar.gz || exit 1
 # https://github.com/Yelp/dumb-init/issues/273
 sed -i '128 i \ \ \ \ packages=[],' dumb-init*/setup.py || exit 1
+# https://github.com/Yelp/dumb-init/issues/286
+echo py >> dumb-init*/requirements-dev.txt
 
 # Replace the versions of python used for testing dumb-init. Since it is
 # testing c code, and not python it shouldn't matter. Also remove the