Use specific ubuntu for py27 tests

[rbikar]

The py2.7 stopped working for GH actions on the latest ubuntu, let's use the older one where py27 still is offered.

[codecov[bot]]

Codecov Report

Base: 97.25% // Head: 97.25% // No change to project coverage :thumbsup:

Coverage data is based on head (a3da31f) compared to base (faac8ad). Patch has no changes to coverable lines.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #200 +/- ## ======================================= Coverage 97.25% 97.25% ======================================= Files 7 7 Lines 1055 1055 ======================================= Hits 1026 1026 Misses 29 29 ``` Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=release-engineering). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=release-engineering)

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[rbikar]

jenkins CI is currently broken