Add upper bound < 5.0.0 to lxml for python 2.7

Support for python 2.7 was removed inhttps://pypi.org/project/lxml/5.1.0/.
However there are no python 2.7 wheels for Windows for lxml >- 5.0.0.

Author: loqs

requirements-optional.txt

@@ -10,4 +10,5 @@ chardet>=2.2.1
 
 # lxml is supported with its own treebuilder ("lxml") and otherwise
 # uses the standard ElementTree support
-lxml>=3.4.0 ; platform_python_implementation == 'CPython'
+lxml>=3.4.0, <5.0.0 ; python_version < '3.0' and platform_python_implementation == 'CPython'
+lxml>=3.4.0 ; python_version >= '3.0' and platform_python_implementation == 'CPython'