Add ubuntu-latest to the matrix of test runners

[pjrobertson]

tests on ubuntu-latest are currently failing, due to the issue in oscrypto: wbond/oscrypto#78 (comment)

Not sure if we want to merge this in, or wait for a solution before we merge

[msramalho]

both https://github.com/wbond/certvalidator
and https://github.com/wbond/oscrypto
seem unmaintained.

the source dependency issue is in certvalidator that installs oscrypto (where the issue is)

alledgedly, this commit (not published to pypi) wbond/oscrypto@d5f3437 solves the issue (not personally verified)

can we fix the oscrypto version to that one even if it is installed as a dependency? that would be a quick fix

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alternative is to find another way to achieve the same as certvalidator is currently doing, that may be a more involved task so I'd put behind the major modularization work we are doing at the moment.

[msramalho]

also I'd not merge this PR until we can make sure the test suite will be working

[pjrobertson]

Sorry, I missed this yesterday. Changes look good, but is this perhaps an opportunity to do away with oscrypto (certvalidator). You said this also fixes the boto3 issue? I checked boto3 and oscrypto isn't a dependency on it.

In my 'remove dependencies' PR I mentioned that the timestamping_enricher has another old module asn1crypto. Perhaps a TODO is to rewrite timestamping_enricher to remove asn1crypto and certvalidator (and subsequently oscrypto). Thoughts? Shall we open an issue on it?