Remove -I for installing from the source in readme

[fireattack]

Per discussion at #5262 (comment), I think we shouldn't by default suggest users to use -I (--ignore-installed) when installing/upgrading from the source.

Doing so causes pip to blindly install a new version without overriding the existing one in the system, and from my experience, it will not update the CLI entry point (a wrapper script usually in path e.g. /Scripts/gallery-dl.exe on Windows) to point to the newly installed version, which defeats the main point of installing the latest dev version.

[mikf]

I think I've added -I back in the day to allow installing a gallery-dl package even when the same version (….dev0) is already installed. Not sure why I didn't choose --force-reinstall instead, maybe that option didn't exist back then.

I think

python3 -m pip install -U --force-reinstall --no-deps https://github.com/mikf/gallery-dl/archive/master.tar.gz

with "Omit --no-deps if requests hasn't been installed yet" as a note would probably be best, or at least better than what is currently recommended by the README.

Speaking of, the current README definitely needs some thorough update, which I've more or less neglected over the last couple years. I'll try to open a PR with some changes over the course of this week to get some feedback.

edit: The failing tests are fixed in 00fe1c8 and bced143

[fireattack]

Thanks! I've updated according to your comment. If you prefer to do these changes yourself in a PR in future instead, feel free to do so, too.