Improve integration testing

[studioph]

From #3:

Expand upon/rework the integration/test projects to cover multiple versions of beartype since that is a core dependency.

Look into using test-pypi or something else for staging/installing the packge into the test project. Right now this is done manually by specifying a path to the built wheel locally which needs to be changed with each version (and is generally annoying)

[studioph]

Currently, the integration pyproject.toml is static - perhaps it would be better to switch to creating it fresh every time with poetry since it's simple and small. This would avoid the need to keep updating the bearboto3 version in the integration pyproject.toml.