Need to pin TF to 1.15.0 instead of 1.15.5 to allow numpy >= 1.21 needed by gymnasium

[discordianfish]

Main reference: https://discuss.tensorflow.org/t/tensorflow-1-15-installation-numpy-dependency/13820

tf 1.15.0 requries 1.16.0 <= numpy < 2.0
tf 1.15.5 requries 1.16.0 <= numpy < 1.19.0

While gymnasium requires numpy >= 1.21

In addition stable baselines only works with TF (not pytorch)

So here the goal is allow an installation of all the supported libraries without conflicts.

SB will be discontinued sooner or later, but all my trained agents are there for now and they are useful for sanity checks and regressions tests