Use html.clean from external project and provide "html_clean" extra d…

…ependency (GH-406)

Following the discussion in https://bugs.launchpad.net/lxml/+bug/1958539, "lxml.html.clean" is now extracted into a separate project "lxml_html_clean":

*    Github: https://github.com/fedora-python/lxml_html_clean
*    PyPI: https://pypi.org/project/lxml-html-clean/
*    Documentation: https://lxml-html-clean.readthedocs.io/en/latest/

The module is available as an "extra" setuptools dependency "lxml[html_clean]", so that:

*    Projects that use lxml without lxml.html.clean will continue to use it without any difference. Except they won't have potentially vulnerable code installed.
*    Projects that need lxml.html.clean will need to switch their requirements from lxml to lxml[html_clean].

The new package is added as a test dependency to continue to test the compatibility between the two projects.

Closes https://bugs.launchpad.net/lxml/+bug/1958539

setup.py

@@ -78,6 +78,7 @@ def static_env_list(name, separator=None):
             'cssselect': 'cssselect>=0.7',
             'html5': 'html5lib',
             'htmlsoup': 'BeautifulSoup4',
+            'html_clean': 'lxml_html_clean',
         }
 
 extra_options.update(setupinfo.extra_setup_args())

setupinfo.py

@@ -20,7 +20,6 @@
     "lxml.builder",
     "lxml._elementpath",
     "lxml.html.diff",
-    "lxml.html.clean",
     "lxml.sax",
 ]
 HEADER_FILES = ['etree.h', 'etree_api.h']