use a limited amount of Python versions for PR runs

[beniwohli]

What does this pull request do?

the CI runs on PRs are getting prohibitively long. To work around
this, we can use only a limited set of Python versions for PR runs
and do a full, nightly run (the same approach we already use for
framework versions).

[apmmachine]

💚 Build Succeeded

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Build stats

• Start Time: 2022-01-11T12:52:00.474+0000

• Duration: 21 min 57 sec

• Commit: 02e17e6

Test stats 🧪

Test	Results
Failed	0
Passed	4809
Skipped	2879
Total	7688

💚 Flaky test report

Tests succeeded.

🤖 GitHub comments

To re-run your PR in the CI, just comment with:

• /test : Re-trigger the build.

• /test linters : Run the Python linters only.

• /test full : Run the full matrix of tests.

• /test benchmark : Run the APM Agent Python benchmarks tests.

• run elasticsearch-ci/docs : Re-trigger the docs validation. (use unformatted text in the comment!)

[v1v]

💯 neat!