CI: Drop the py27

[LinglingXiang]

@rbikar , please help review it, thanks

[rbikar]

@LinglingXiang the CI needs fix because of problematic installation of rpm-py-installer, can you create PR with fix or add it even to this PR? More info: https://github.com/release-engineering/pushsource/pull/332

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (d8f76e8) 96.58% compared to head (306d393) 96.58%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #203 +/- ## ======================================= Coverage 96.58% 96.58% ======================================= Files 7 7 Lines 850 850 ======================================= Hits 821 821 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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[LinglingXiang]

@rbikar , please help review again, thanks