I was unable tp build calibre at the current git master as it relies on
pytthon27Packages.html2text. Python 2.7 support was removed in the
latest version. This PR downgrades html2text when using an unsupported
Python version.
This PR does not fix building python37Packages.html2text
and python38Packages.html2text, which were broken at master due to what
seems to be a problem in buildPythonPackage.
