PyGObject, venv and Ubuntu 20.10: ImportError: libffi.so.7: cannot open shared object file: No such file or directory

After updating from Ubuntu 20.04 to Ubuntu 20.10 I am not able to run PyGObject applications on a Python Virtual Environment.

I can reproduce the issue following these simple steps:

sudo apt-get install python3-venv
python3 -m venv venv
source env/bin/activate
pip install PyGObject==3.38.0
python3 hw.py

Where hw.py is the standard PyGObject Hello World:

#!/usr/bin/env python3

import gi
gi.require_version("Gtk", "3.0")
from gi.repository import Gtk

window = Gtk.Window(title="Hello World")
window.show()
window.connect("destroy", Gtk.main_quit)
Gtk.main()

When I try to run it, it throws this error:

$ python3 hw.py 
Traceback (most recent call last):
  File "hw.py", line 3, in <module>
    import gi
  File "/home/leinardi/temp/hw/venv/lib/python3.8/site-packages/gi/__init__.py", line 40, in <module>
    from . import _gi
ImportError: libffi.so.7: cannot open shared object file: No such file or directory

Running hw.py on the System Environment works correctly (the issue only happens when using the Virtual Environment).

A workaround is to find and provide a copy of libffi.so.7 in /usr/lib/x86_64-linux-gnu/ but, since this library version is not available inside the official repositories of Ubuntu 20.10, this is a difficult and unsafe operation.