Resolve problems with anyio 4

[dvarrazzo]

Anyio 4 has incompatibilities so, when it was released, tests were broken. Because we were still maintaining Python 3.7 and Anyio 4 doesn't support it, I pinned the version we depend on to < 4 (e847f3c).

Now we dropped Python 3.7 support in the master branch (02dfd08) so we can remove the upper boundary. However I think tests still need to be fixed, because they use a feature that got refactored.

I don't know if it's feasible to depend on any anyio version or if the only way to support 4 is to drop 3, which is a second choice I'd be ok with, if necessary.