ok...
thats because unittest2 is in the "test" extra of extras_require. IMO, it makes no sense to install unittest2 on production environments where not tests will ever run. on development machines you can install it like so:
$ pip install "icalendar [test]"

assuming your pip installation isn't years-old (https://github.com/pypa/pip/issues/7)

the above doesn't work when installing directly from a git repository.