Fix #488: append document-child comments in document order in lxml

[gsnedders]

Fixes #488. Note that this still needs tests.

[codecov-commenter]

Codecov Report

Merging #496 into master will decrease coverage by 0.09%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #496      +/-   ##
==========================================
- Coverage   90.91%   90.81%   -0.10%     
==========================================
  Files          49       50       +1     
  Lines        6933     6969      +36     
  Branches     1321     1327       +6     
==========================================
+ Hits         6303     6329      +26     
- Misses        472      479       +7     
- Partials      158      161       +3     

Impacted Files	Coverage Δ
html5lib/treebuilders/etree_lxml.py	79.36% <50.00%> (-0.56%)	⬇️
html5lib/tests/tree_construction.py	94.16% <0.00%> (-1.40%)	⬇️
html5lib/tests/test_sanitizer.py	94.59% <0.00%> (-0.15%)	⬇️
html5lib/tests/test_serializer.py	93.05% <0.00%> (-0.05%)	⬇️
html5lib/tests/test_treewalkers.py	97.14% <0.00%> (-0.05%)	⬇️
html5lib/_trie/__init__.py	100.00% <0.00%> (ø)
html5lib/tests/test_encoding.py	100.00% <0.00%> (ø)
html5lib/_trie/datrie.py	74.19% <0.00%> (ø)
html5lib/tests/tokenizer.py	83.33% <0.00%> (+1.23%)	⬆️

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