springer-dl: download the set of books Springer released for free during the
2020 COVID-19 outbreak

- bypasses Google captcha
- supports PDF and EPUB book formats
- sorts by title, includes ISBN in filename
- no external dependencies

usage: springer_dl.py [-h] --path PATH

optional arguments:
  -h, --help   show this help message and exit
  --path PATH  path to download books

$ ls -R books/
'books/Understanding Statistics Using R':
'Understanding Statistics Using R - 978-1-4614-6227-9.epub'
'Understanding Statistics Using R - 978-1-4614-6227-9.pdf'