HTTPSPublicKeyRetriever caches asyncio event loop - pytest-asyncio issues

Hey there. I'm using `atlassian_jwt_auth` in an ayncio context; specifically using [pytest-asyncio](https://pypi.org/project/pytest-asyncio/) for the event loop creation. By default this library creates a new event loop for each test. (This is my desired behavior.)

However when running two tests that both utilize the HTTPSPublicKeyRetriever, the second test fails because the cached `HTTPSPublicKeyRetriever._class_session` is used with a now-shutdown event loop.

I've gotten around this with a fixture like so:

```python
@pytest.fixture(autouse=True)
async def clean_https_session():
yield
await HTTPSPublicKeyRetriever._class_session.close()
HTTPSPublicKeyRetriever._class_session = None
```

However this feels sort of wacky. Ideally I'd like the library to detect this and invalidate the _class_session either when the loop is shut down or when the session is gotten.

Adding a conditional [here](https://github.com/atlassian/asap-authentication-python/blob/master/atlassian_jwt_auth/contrib/aiohttp/key.py#L23-L26) of `client.loop != self.loop` fixes the issue, but `client.loop` is [deprecated in aiohttp](https://github.com/aio-libs/aiohttp/blob/v3.7.4.post0/aiohttp/client.py#L995-L1001) and actually has been removed in the newest master branch.

Beginne in atlassian_jwt_auth/contrib/aiohttp/key.py bei der Sitzungsverwaltung von HTTPSPublicKeyRetriever und reproduziere anschließend den Fehler mit zwei pytest-asyncio-Tests, die separate Event-Loops verwenden. Ermittle, wie der zwischengespeicherte _class_session ungültig gemacht werden kann, ohne auf das veraltete client.loop-Attribut angewiesen zu sein; abgeschlossen ist die Aufgabe, wenn beide Tests beendet werden, ohne eine an eine geschlossene Schleife gebundene Session wiederzuverwenden.

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