mail-client/balsa: fix html2text handling for HTML mail quoting

mail-client/balsa: fix html2text handling for HTML mail quoting

configure first checks for python-html2text binary, for which we had a
dev-python/html2text dependency. But our pyhtml2text package installs
a pyhtml2text binary instead, thus this will not satisfy the check,
and HTML quoting support via a html2text binary gets disabled.
Instead of tring to make it work with pyhtml2text, just depend on
app-misc/html2text instead, which as a bonus has alpha keywords.
This will make the balsa check pass with /usr/bin/html2text and HTML
quoting should work (at least however good html2text over pyhtml2text is).

Bug: https://bugs.gentoo.org/660322
Bug: https://bugs.gentoo.org/626690
Package-Manager: Portage-2.3.40, Repoman-2.3.9