Bump lxml version to latest

lxml changelog (https://lxml.de/5.4/changes-5.4.0.html)
5.4.0 bugs fixed (2025-04-22)
Binary wheels use libxml2 2.13.8 and libxslt 1.1.43 to resolve several CVEs. (Binary wheels for Windows continue to use a patched libxml2 2.11.9 and libxslt 1.1.39.)

5.3.2 bugs fixed (2025-04-05)
This release resolves CVE-2025-24928 as described in https://gitlab.gnome.org/GNOME/libxml2/-/issues/847
Binary wheels use libxml2 2.12.10 and libxslt 1.1.42.
Binary wheels for Windows use a patched libxml2 2.11.9 and libxslt 1.1.39.

5.3.1 bugs fixed (2025-02-09)
GH#440: Some tests were adapted for libxml2 2.14.0. Patch by Nick Wellnhofer.
LP#2097175: DTD(external_id="…") erroneously required a byte string as ID value.
GH#450: iterparse() internally triggered the DeprecationWarning` added in lxml 5.3.0 when parsing HTML.

Author: paulhiggs

tests/requirements.txt

@@ -1,2 +1,2 @@
-lxml==5.3.0
+lxml==5.4.0
 requests==2.32.4