Use Sentry internal PyPI server in CI tests

[antonpirker]

Problem Statement

Tests could be faster. Maybe using the Sentry internal PyPI server can speed up things.

Solution Brainstorm

Sentry has an internal PYPI server (https://github.com/getsentry/pypi)

(Employees can see internal announcement here: https://vanguard.getsentry.net/p/cl7rq4a4o50570ls6ln2lspye)

Add all the dependencies of our tests (old versions of a lot of frameworks) to the internal PyPI server and then use this server in the test suite run in CI.

Measure if this improves performance, if it does not, revert back to public PyPI server.