Remove cython/setuptools version checks since setup will install them.

The `setup` call specifies the necessary versions of cython and setuptools so when you pip install it will install them automatically:

https://github.com/jparyani/pycapnp/blob/4a11ffedd943de71a6dce8976c243806a1cafbd1/setup.py#L123

So the explicit checks at the top of `setup.py` are not necessary and cause the pip installation to fail rather than install the necessary versions.