Add pytest-sentry for pytest runs

[drew2a]

This PR helps us to collect statistics about flaky tests.

Let's say you have a testsuite with some flaky tests that randomly break your CI build due to network issues, race conditions or other stuff that you don't want to fix immediately. The known workaround is to retry those tests automatically, for example using pytest-rerunfailures.
One concern against plugins like this is that they just hide the bugs in your testsuite or even other code. After all your CI build is green and your code probably works most of the time.
pytest-sentry tries to make that choice a bit easier by tracking flaky test failures in a place separate from your build status. Sentry is already a good choice for keeping tabs on all kinds of errors, important or not, in production, so let's try to use it in testsuites too.

For the pytest-sentry it is necessary to specify PYTEST_SENTRY_DSN.
There are several options for implementing it in our GitHub Actions. We discussed all of them within the dev team and came to a conclusion to store the PYTEST_SENTRY_DSN as a repository secret.
For the secret to be accessible from !PR.yml it is necessary to change the environment in which this workflow runs from unprivileged to privileged (from pull_request to pull_request_target). To protect access from the forks PR to the main secrets, the gatekeeper job was introduced.
It is only possible to pass the gatekeeper for users with the "write" (or higher) permission.

In the case, the user doesn't have write access, the gatekeeper will not allow the user to perform some of the tests:

Related:

• [Tests] test_close_process_errors. ProcessLookupError: [Errno 3] assume no such process  #7164
• [Tests] Timeout in pytest tests #7134
• [Tests] TestRemoteQueryCommunity.test_process_rpc_query_match_many OSError: [Errno 9] Bad file descriptor #7111
• [Tests] sqlite3.OperationalError: disk I/O error #7063

Refs:

• https://github.com/getsentry/pytest-sentry
• https://nathandavison.com/blog/github-actions-and-the-threat-of-malicious-pull-requests
• https://blog.gitguardian.com/github-actions-security-cheat-sheet/
• https://securitylab.github.com/research/github-actions-preventing-pwn-requests/
• https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#pull_request_target
• https://github.com/lannonbr/repo-permission-check-action
• https://docs.github.com/en/actions/using-workflows/reusing-workflows#passing-inputs-and-secrets-to-a-reusable-workflow

[xoriole]

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