s3 download extended errors

[dorotamierzwa]

Goals:

1. add adjustment to s3_download_data function
2. Add Python version in requirements.txt

AC:

• s3_download_data includes restrictions regarding accepted separator values (not possible to pass ints)
• s3_download_data handles cases when a file is empty (both when its a single file or any of the files included in a prefix, with an option skip_empty_files how to handle them)
• when running pip install -r requirements.txt, no higher Python version is installed than 3.7.5