drop xhtml content-type support (closes #2065651) * HTMLStream does not care about xml any more * reqquest.demote_to_html and .xhtml_browser are deprecated * web config: drop force-html-content-type option * adjust tests

from os.path import join
assert __file__.endswith(join('scripts', 'script2.py')), __file__
assert '__main__' == __name__, __name__
assert ['-v'] == __args__, __args__