No module named processlifetime

I have an (old) plone 5 site that suddenly would not start anymore. Nothing was done on purpose (but I ran buildout again to try to get it up again), but note that I have other sites running new(er) Python.

Any suggestion on how to sort this out (install just for 2.7) ?

  /instance8015/zinstance$ bin/instance fg
  Traceback (most recent call last):
   File "/path/to/instance8015/zinstance/parts/instance/bin/interpreter", line 342, in <module>
	 exec(compile(__file__f.read(), __file__, "exec"))
   File "/path/to/instance8015/buildout-cache/eggs/Zope2-2.13.26-py2.7.egg/Zope2/Startup/run.py", line 76, in <module>
	 run()
   File "/path/to/instance8015/buildout-cache/eggs/Zope2-2.13.26-py2.7.egg/Zope2/Startup/run.py", line 17, in run
	 import Zope2.Startup
   File "/path/to/instance8015/buildout-cache/eggs/Zope2-2.13.26-py2.7.egg/Zope2/__init__.py", line 60, in <module>
	 from Zope2.Startup.run import configure
   File "/path/to/instance8015/buildout-cache/eggs/Zope2-2.13.26-py2.7.egg/Zope2/Startup/__init__.py", line 28, in <module>
	 from zope.processlifetime import ProcessStarting

bin/pip2.7 install zope.processlifetime
Requirement already satisfied: zope.processlifetime in ./lib/python2.7/site-packages (2.3.0)
Requirement already satisfied: setuptools in ./lib/python2.7/site-packages (from zope.processlifetime) (40.2.0)
Requirement already satisfied: zope.interface in ./lib/python2.7/site-packages (from zope.processlifetime) (5.4.0)

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