Remove unused dependencies from requirements.txt

[yezyilomo]

Thank you for the great library, I really appreciate the work you are doing.
The setup.py shows that the package has only two dependencies https://github.com/Kalebu/pypesa/blob/d2526183b09de997307b200530af4baab749b992/setup.py#L34-L39 But in requirements.txt there’re way more dependencies, is there any reason for that.

[Kalebu]

Thanks for pointing that out, I think I was out of the virtual environment when I freeze requirements, I'm gonna update accordingly