Build our own copy of lxml using Ubuntu's libxml2/libxslt

[Bjwebb]

(don't use the prebuilt wheel)

https://opendataservices.plan.io/issues/36790

[coveralls]

Pull Request Test Coverage Report for Build 3985845392

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 71.579%

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Totals
Change from base Build 3732893922:	0.0%
Covered Lines:	476
Relevant Lines:	665

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💛 - Coveralls

[odscjames]

--no-binary lxml==4.9.1

My only comment would be, is it possible to do

--no-binary lxml

As otherwise I fear it'll be far to easy for version numbers to get out of sync in the future. Or maybe put a comment in requirements.txt, if that preserves comments between builds.

If not no worries - and thanks

[Bjwebb]

I thought that wouldn't work for some reason, but it clearly does, so I've made the change.