CI doesn't support 2.7 anymore

[sentrivana]

How do you use Sentry?

Sentry Saas (sentry.io)

Version

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Steps to Reproduce

Open a PR and see that the 2.7 tests fail with

  Warning: The support for python 2.7 will be removed on June 19. Related issue: https://github.com/actions/setup-python/issues/672
  Version 2.7 was not found in the local cache
  Error: The version '2.7' with architecture 'x64' was not found for Ubuntu 20.04.

E.g. here: https://github.com/getsentry/sentry-python/actions/runs/5311404601/jobs/9614596733?pr=2177

Reason (and possible workarounds): actions/setup-python#672

TODO

• see if we can simply use an older Ubuntu version
• if not, we can try running all 2.7 suites in a container

Expected Result

CI for 2.7 still works.

Actual Result

CI for 2.7 doesn't work.

[antonpirker]

We probably can just select another Ubuntu version in the test matrix: https://github.com/getsentry/sentry-python/blob/master/.github/workflows/test-integration-django.yml#L33-L39