fix #464 with align httpx_client AsyncOAuth2Client.stream to httpx._c…

[bjoernmeier]

…lient.AsyncClient.stream

Fix for #464 .

The method httpx._client.AsyncClient.stream is an asynccontextmanager
and httpx._client.AsyncClient.stream should as well so both clients
allow interchangeability.

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What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes as it changes the interface again, which was broken (by accident?) in 1.0.0
• No

If yes, please describe the impact and migration path for existing applications:

AsyncOAuth2Client.strema needs to be sued like this:

client = AsyncOAuth2Client(...)
async with client.stream("GET", 'https://i.b') as stream:
    async for chunk in response.aiter_bytes():
        # do something

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[lepture]

Thanks