Updates to required packages

[jrsacher]

Updated:

• selfies from 1.0.1 to 2.1.0. Note: this version only supports Python >= 3.7. If compatibility with previous versions is required, this should be changed to 2.0.0.
• crem from 0.2.4 to 0.2.9
• loguru from 0.4.1 to 0.6.0

Added:

• pandas>=1.0.* to requirements.txt to match with requires.txt, setup.py

Incremented version to 2.9.1

I did a bit of testing and nothing broke. That said, it'd be great if some more thorough testing was done.