LXML should respect case of infinity and NaN for XML Schema conformance

[Haxtibal]

lxml.objectify XSI-annotates Python float as xsd:double. W3Cs XML Schema Datatype specification for double [0] says:

> The special values positive and negative infinity and not-a-number have lexical representations INF, -INF and NaN, respectively.

Thus case matters
- "INF","-INF", "NaN" are valid
- but lxml.objectify generates "inf", "-inf", "nan", which are invalid

Reproducer:

from lxml import etree, objectify
root = objectify.ElementMaker(annotate=False).test(float("inf"))
objectify.xsiannotate(root)
print(etree.tostring(root, pretty=True).decode())

gives

<test xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="xsd:double">inf</test>

xmllint (as well as multiple other schema validators I've tested) will correctly detect the error:

Schemas validity error : Element 'test': 'inf' is not a valid value of the atomic type 'xs:double'.

[0] https://www.w3.org/TR/xmlschema-2/#double

lxml.etree : (4, 7, 1, 0)
libxml used : (2, 9, 4)
libxml compiled : (2, 9, 4)
libxslt used : (1, 1, 33)
libxslt compiled : (1, 1, 33)

[scoder (scoder)]

Fixed in https://github.com/lxml/lxml/pull/338

[scoder (scoder)]

Merged in https://github.com/lxml/lxml/commit/f7bb07b5f68fede97754685dad076cd7b7442bac