biosppy requirement missing from requirements.txt

[borisbaldassari]

Following our meeting on 2021-09-21, after installing all requirements in a clean environment, dependency biosppy was missing.
It should be added to requirements.txt.

Note: that was on the master branch.

[borisbaldassari]

I'll propose a PR for that, with my set of requirements (many modules updated probably), including biosppy.