Flickr-download

This release has a few speed improvements that especially will speed up things when downloading large sets, and having to restart because the Flickr API inevitably errors out at some point:

* You can now add the `--metadata` flag to have the script keep track of which files have been downloaded in a way that eliminates the API call to Flickr entirely for an already downloaded image
* The `--cache` flag now also caches the set list
* One extraneous API call per image download removed

And then a bunch of fixes:
* Actually iterate through all photosets, photos, etc (and not only the first 500)
* Handle error where photo date is too old for the underlying OS to handle

Behind the scenes, there are now a few more tests and everything is type annotated.

A much delayed update to Python 3....

It also switches the build system to use [Python Poetry](https://python-poetry.org/) and includes the latest version (0.7.4) of the [Flickr API library](https://github.com/alexis-mignon/python-flickr-api).

* A couple of great documentation updates by IzakMarais
* Fixes to various issues when downloading by astanin

Thanks to both for the contributions!

* Now sets the right filename extension for originals (when not `.jpg`)
* Adds ability to skip downloads of the photos (`-o`)
* Removes some unnecessary API calls

Upgraded to use newest version of PyYAML

Script now handles old 90 seconds videos (hello 2008!), and continues on IO Errors instead of throwing an exception and stopping.