Remove fastapi version upper limit

[hillairet]

For some reason the github action for python 3.9 and 3.10 would just hang if a fastapi versoion 0.69 and above was used. See #85 for the tests.

This must be related somehow to the introduction of Trio in fastapi 0.69 somehow.

[hillairet]

Also I can reproduce it locally.

1. Remove the upper limit on the fastapi dependency in pyproject.toml
2. Then update the dependencies to have a fastapi > 0.68: poetry update
3. Now just run the pytest:

poetry shell
tox