feat(tests): Add a Starlette testcase that uses Uvicorn

[sentrivana]

Follow up to detect regressions like https://github.com/getsentry/sentry-python/issues/3021. Verified that the new test fails if I reintroduce the async ensure decorator.

The new Uvicorn fixture might be also useful for other tests.

Also took the liberty to update the Starlette versions in Tox.

Relates https://github.com/getsentry/sentry-python/issues/2892

WIP: works locally. freezes in ci.

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[codecov[bot]]

:x: 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
13523	1	13522	4121

View the top 1 failed tests by shortest run time

tests.integrations.stdlib.test_httplib test_http_timeout

Stack Traces | 61.2s run time

.../integrations/stdlib/test_httplib.py:373: in test_http_timeout
    assert len(transaction["spans"]) == 1
E   assert 6 == 1
E     +6
E     -1

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