requirements.txt: ERROR: No matching distribution found for ccxt==1.66.32

## ERROR: Could not find a version that satisfies the requirement ccxt==1.66.32 (from finrl)

This error is produces when installing finrl via: ``!pip install git+https://github.com/AI4Finance-Foundation/FinRL.git`` in jupyter notebooks, e.g.: ``https://github.com/AI4Finance-Foundation/FinRL-Tutorials/blob/master/1-Introduction/FinRL_PortfolioAllocation_NeurIPS_2020.ipynb``

__This error can be solved by changing line in ``requirements.txt`` from ``ccxt==1.66.32 `` to ``ccxt>=1.66.32``.__

That quick fix is written in the comment. Why is it not already fixed?