A lot of people want to treat copyright licenses as code, but law doesn't work like that. The question is what a court/judge would think about the work as a whole and how it functions.
If you gave a non-technical person a copy, would they look at the project as a single work or two work that simply interact with each other. If it is the later and you think that is what most people would interpret it as, then no need to change the license. If not then you should change the license as what you got is a combined work which GPLv3 has specific requirements for.
GPL is "viral". If your code has inside a copy (perhaps with some modifications) of a project that has the GPL license, then you must use the GPL license for the whole project. https://en.wikipedia.org/wiki/Viral_license
That is the reason why some people love the GPL license.
That is the reason why some people hate the GPL license.
Yes. The license is GPL not LGPL so there is no other way to interpret the situation.
Even if you use locally installed version of phash library, you still must license under GPLv3. They way the library package is installed has nothing to do with the license you must choose.
Vivaldi is BSD with GPL Chromium code.
What I know about this is from 10 years ago. It's one of the "Twenty questions about the GPL" listed at https://jacobian.org/2009/jul/13/gpl-questions/ .
One of the responses is at https://gist.github.com/Neurogami/146713 :
> "Unfortunately all the FSF can provide on a number of these questions is their opinion on the matter. Having spoken with the Software Freedom Law Center about the import issue recently the reality is there is no answer to some of these questions because no one has ever taken them to court."
The HN thread about those 20 questions is at https://news.ycombinator.com/item?id=702304 .
My view is to follow the intent of GPLv3 and either make your package GPLv3 (though you can leave the copyright to your own code as MIT), or remove the GPLv3 package.