deps: bump minimal lxml version to 5.0.0

It include libxml2 2.12.x which heavily improves parser performance and
reduces memory usage. It also behaves better on parsing huge files as
discovered in translate/translate#4302

requirements.txt

@@ -24,7 +24,7 @@ hiredis>=1.0.1,<2.4
 html2text>=2019.8.11,<2020.1.17
 iniparse==0.5
 jsonschema>=4.5,<5
-lxml>=4.9.1,<5.1
+lxml>=5.0.0,<5.1
 misaka>=2.1.0,<2.2
 mistletoe>=1.1.0,<1.3
 nh3>=0.2.14,<0.3