[pixiv] use api for illust series

[mix5003]

i think this should fix can not download R-18 series in pixiv

for illust_series api i just guess url and lucky to find it.

[mix5003]

@mikf i didn't realize_pagination.

if use _pagination should it remove offset from illust_series?

[mikf]

No, it's fine the way it is.

Explaining some unorthodox method like

gallery-dl/gallery_dl/extractor/pixiv.py

Lines 1007 to 1009 in 8cd625f

	if key_data:
	self.data = data.get(key_data)
	key_data = None

to get illust_series_detail data into the paginated loop is far more work than just implementing it myself (at least for me), so I did just that.

for illust_series api i just guess url and lucky to find it.

Nice. Well done. And thank you.