Add absl-py to requirements.txt

[intermezzio]

When importing finetune, the module 'absl' is not found unless it's installed manually.

Setup Information
Relevant Packages:

finetune==0.8.6

Reproduce with

import finetune

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/<USERNAME>/anaconda3/envs/<ENV>/lib/python3.8/site-packages/finetune-0.8.6-py3.8.egg/finetune/__init__.py", line 4, in <module>
    import absl.logging
ModuleNotFoundError: No module named 'absl'