Upgrade to Python3

[simitt]

Motivation/summary

This PR upgrades APM Server to use Python 3. After elastic/beats#14798 has landed upgrading the beats framework changes python dependencies e.g. in the Makefile from python2 to python3.
In #3001 we agreed on not spending time on upgrading scripts and the system tests to python3. Syncing the latest beats framework showed that we would need to completely get rid of Makefile dependencies to avoid python version conflicts. The other option would have been to not upgrade the tests, but still upgrade the tooling, which would require handling two python versions.

Therefore I decided to look into what's necessary to actually upgrade the python version, as changing the Makefile and ensuring everything works as expected also costs time. Turns out that only a few adoptions in the system tests were necessary (took me less than 2 hours to upgrade).

So this PR contains

• changes to the make update-beats script to sync the test files
• updates to the latest beats framework
• upgrading system tests to python3: main changes were related to testing TLS configurations, please have a closer look at that when reviewing
• upgrading APM Server to python3

Checklist

- [ ] I have signed the Contributor License Agreement.

• My code follows the style guidelines of this project (run make check-full for static code checks and linting)
• I have rebased my changes on top of the latest master branch

• I have made corresponding changes to the documentation
  - [ ] I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

- [ ] I have updated CHANGELOG.asciidoc

How to test these changes

CI passing should test a huge part of this. One-off scripts need to be tested manually.

Related issues

#3001

[Bamieh]

Codecov Report

Merging #3357 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3357   +/-   ##
=======================================
  Coverage   79.36%   79.36%           
=======================================
  Files         108      108           
  Lines        5698     5698           
=======================================
  Hits         4522     4522           
  Misses       1176     1176           

[simitt]

I did some manual testing for the tooling and as far as I could see everything works as expected.

[axw]

Looks good, thank you. Only thing I'm not too sure about is the Windows CI bit. It's fine, but are we installing in the right place? How was Python being installed before?

Also, there appears to be an issue with integration tests, it's complaining about python3-venv.

[simitt]

Only thing I'm not too sure about is the Windows CI bit. It's fine, but are we installing in the right place? How was Python being installed before?

I went along with the according changes in beats, mainly because I wouldn't know how else to solve this, and didn't think this is problematic.

The failing integration test setup is concerning, thanks for pointing that out, will look into it.

[simitt]

Required changes for APM Integration Tests are addressed in elastic/apm-integration-testing#766.

[simitt]

We usually don't git rebase on PRs, but since it makes sense to keep multiple commits here and they should be squashed I am rebasing this one.