Remove support for Python 2

[rst0git]

This pull request removes support for Python 2 in CRIU and disables the CI tests for CentOS 7, as we discussed in https://github.com/checkpoint-restore/criu/issues/2203.

[codecov-commenter]

Codecov Report

Patch coverage: 68.42% and project coverage change: -0.32 :warning:

Comparison is base (e6427c5) 70.72% compared to head (06ca563) 70.41%.

:exclamation: Current head 06ca563 differs from pull request most recent head 6d6db09. Consider uploading reports for the commit 6d6db09 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## criu-dev #2210 +/- ## ============================================ - Coverage 70.72% 70.41% -0.32% ============================================ Files 133 133 Lines 33248 33392 +144 ============================================ - Hits 23514 23512 -2 - Misses 9734 9880 +146 ``` | [Impacted Files](https://app.codecov.io/gh/checkpoint-restore/criu/pull/2210?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | Coverage Δ | | |---|---|---| | [criu/cr-check.c](https://app.codecov.io/gh/checkpoint-restore/criu/pull/2210?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-Y3JpdS9jci1jaGVjay5j) | `61.58% <0.00%> (-0.08%)` | :arrow_down: | | [criu/libnetlink.c](https://app.codecov.io/gh/checkpoint-restore/criu/pull/2210?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-Y3JpdS9saWJuZXRsaW5rLmM=) | `75.26% <ø> (-0.27%)` | :arrow_down: | | [criu/pipes.c](https://app.codecov.io/gh/checkpoint-restore/criu/pull/2210?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-Y3JpdS9waXBlcy5j) | `82.44% <0.00%> (ø)` | | | [criu/kerndat.c](https://app.codecov.io/gh/checkpoint-restore/criu/pull/2210?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-Y3JpdS9rZXJuZGF0LmM=) | `57.68% <76.47%> (ø)` | | ... and [21 files with indirect coverage changes](https://app.codecov.io/gh/checkpoint-restore/criu/pull/2210/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None)

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[adrianreber]

This is a huge change, but it looks correct. Not entirely sure if this might break packaging CRIU in some places. Hard to tell.

[rst0git]

This is a huge change, but it looks correct.

Indeed, these changes clean-up Python 2 compatibility fixes we have introduced over the past few years.

Not entirely sure if this might break packaging CRIU in some places. Hard to tell.

I don't expect these changes to break packaging as we have already added all necessary build dependencies for Python 3 in the 3.18 release.