fix create_memory_object_stream not subscriptable at runtime in old anyio versions

[graingert]

Summary

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

[graingert]

@Kludex probably this needs a test matrix for each anyio version you want to support?

[Kludex]

Were you able to test against anyio 3.4.0?

[graingert]

3.6.2 is the oldest version I could get running

[Kludex]

We should change the pinned version then

[graingert]

@Kludex bumped