All `base_url` arguments must be optional

[makukha]

All base_url and base_href arguments have default value of None, but not all of them are matked as Optional.

Functions and methods affected:

• lxml.etree.fromstring
• lxml.etree.parse
• lxml.html.HtmlMixin.make_links_absolute
• lxml.html.HtmlMixin.rewrite_links
• lxml.html.fragments_fromstring
• lxml.html.fromstring
• lxml.objectify.fromstring