The python binding to libapt-pkg does not offer any way to do the rough
equivalent of apt-get's "upgrade --with-new-pkgs".

The Cache.upgade method allows doing a "full/dist upgrade" or a
"partial/safe upgrade":

  Cache.upgrade(self, dist_upgrade: bool = False) -> None

That forwards the call to apt_pkg.DepCache.upgrade which is implemented
by PkgDepCacheUpgrade:

  static PyObject *PkgDepCacheUpgrade(PyObject *Self,PyObject *Args)
  {
     [...]
     char distUpgrade=0;
     if (PyArg_ParseTuple(Args,"|b",&distUpgrade) == 0)
        return 0;
     [...]
     if (distUpgrade)
        res = APT::Upgrade::Upgrade(*depcache, 0);
     else
        res = APT::Upgrade::Upgrade(*depcache, APT::Upgrade::FORBID_REMOVE_PACKAGES |
                                               APT::Upgrade::FORBID_INSTALL_NEW_PACKAGES);
     [...]
  }

What's missing is allowing python to call Upgrade with
FORBID_REMOVE_PACKAGES but without FORBID_INSTALL_NEW_PACKAGES
(pseudocode):

     int mode_flags = (upgrade_mode == FULL)
        ? 0
        : (upgrade_mode == WITH_NEW_PKGS)
           ? APT::Upgrade::FORBID_REMOVE_PACKAGES
           : APT::Upgrade::FORBID_REMOVE_PACKAGES |
             APT::Upgrade::FORBID_INSTALL_NEW_PACKAGES;
     res = APT::Upgrade::Upgrade(*depcache, mode_flags);

Plus the API adjustments to expose this feature.