Python and dependency versions

[hatlessman]

Hello! Could you update the docs with info about using python==3.8 making things easier (praat-parselmouth only distributes binaries for python 3.8 and I couldn't get it to compile), numba==0.49 (numba.decorators was pulled in 0.50) , resampy==0.3.1 (to match numba==0.49), and add python_speech_features, pandas, and tensorflow to requirements.txt?