At the end, desperate, I managed to do it myself:

import ete3
from ete3 import Tree
t = Tree('(((A,C),((H,F),(L,M))),((B,(J,K)),(E,D)));')
# to get all possible rooting on terminal branches
ls = []
for leaf in t:
    ls.appendleaf.name)    # add a parenthesis before leaf 
for i in ls:
    t.set_outgroup(t&i)
    t.write(format=1, outfile="myfolder/"+ i + ".nwk")

# to get all possible rooting on internal branches
edge = 0
ancestor = []
for node in t.traverse():
    if not node.is_leaf():
        if not node.is_root():
            node.name = "NODE_%d" %edge
            ancestor.appendnode.name)      # add a parenthesis after append
            edge += 1
for i in ancestor:
    t.set_outgroup( t&i )
t.write(format=1, outfile="myfolder/"+ i + ".nwk")

In Biopython, using the module Bio.Phylo, you can reroot a tree object using the method root_with_outgroup.

Use this script when you do not have duplicated labels in your tree, otherwise when it generates outgroups by terminal tips it will overwrite the one with the same name per terminal tip. You can change the format of the trees to be written as you like (according to ETE3 format rules).