Remove test_minor_version_does_not_reconfigure_wipe() because when run
during dev cycle that test reconfigure with .99 -> .100 which is
considered a major version change now. It is covered by a more efficient
internal test now anyway.
While at it, remove no-op `with Path(self.builddir):` statement, the
intention was clearly to set workdir.
Fixes: #9260
This simplifies things for us, as we don't have to have threading
imported for no other reason, and we can remove the
`an_unpicklable_object` from the Interpreter and mesonlib, since there
was only one user of this.