remove dependency upper bounds in pyproject.toml

The package pyproject toml is using `^` limit some packages which is causing versioning conflicts. 
`^` in poetry prevents changes on the leftmost non-zero digit. In the case of, for example, pampy = "^0.2.1", we're stuck with the minor revision `0.2.x` and only have wiggle room to change the patch versions; which is problematic.
Setting some packages to `>=` could be useful. 