logilab-mtconverter #268147 remove embedded copy of html2text [validation pending]

it exists as a separate library, isn't our code, and the external version has the good idea of being python3-compatible.

prioritynormal
typetask
done in0.9.0
load0.500
load left0.000
closed by#fda99736b65f [transforms] Remove bundled copy of html2text
patch[transforms] Remove bundled copy of html2text [applied]