deps: loosen `nbclient` dependency version bounds

[NicolasT]

The current bounds, ~=0.5.10, translate to >= 0.5.10, == 0.5.* according to the "Version specifiers" spec. Given nbclient is currently at version 0.10.0, loosening the bounds allows for pytest-notebook to be used in environments using newer versions of nbclient.

See: https://packaging.python.org/en/latest/specifications/version-specifiers/#compatible-release