Add sympy and numexpr

sympy : I use it in my code for error propagation.

numexpr : For evaluating formulae without ROOT.

ideally, these would go also to default/2021-02-03, because I need python 3.8 to unpickle some files. Unpickling seems not to be possible/easy in 3.9, if the files were pickled in 3.8, due to some ROOT pythonization problem.