dune-solvers

Let pip decide whether a new version should be installed or the current
one is okay.  This ensures that the virtual environment is updated
(either upgraded or downgraded) whenever a new version of a package is
requested.

The hardest part here is figuring out if a package is installed in
the venv (which also has to be done twice to account for the presence
of either setuptools in Python <3.8, or importlib in Python >=3.8).

Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Cc: John Snow <jsnow@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>