Use ASCII case folding for :lang() and attribute values

[itecz26]

#200 moved element and attribute names to ASCII case folding, but two translation sites still fold the selector side of a comparison with full Unicode str.lower() while the document side only folds A-Z: the :lang() argument in HTMLTranslator.xpath_lang_function(), and attribute values under the lower_case_attribute_values extension point. str.lower() maps exactly two non-ASCII code points into ASCII (U+212A KELVIN SIGN to "k", U+0130 to "i" plus U+0307), so a :lang() argument containing U+212A compiles to starts-with(..., 'k-') and matches lang="K" or lang="k", while an element whose lang is actually U+212A is missed:

>>> from lxml.html import fromstring
>>> from cssselect import HTMLTranslator
>>> doc = fromstring('<p lang="K">x</p>')
>>> doc.xpath(HTMLTranslator().css_to_xpath(":lang(\u212a)"))
[<Element p at 0x...>]

Language-range matching is ASCII case-insensitive (RFC 4647 section 3.3.1, and the HTML specification's definition of :lang), so the argument must be folded the same way as the translate() already emitted for the @lang side. Same reasoning for attribute values: that knob's contract is that the document parser lower-cased the values, an ASCII transformation.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.79%. Comparing base (99ec093) to head (51ca8bf).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #201   +/-   ##
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  Coverage   99.79%   99.79%           
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  Files           3        3           
  Lines         987      987           
  Branches      161      161           
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  Hits          985      985           
  Misses          1        1           
  Partials        1        1           

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