consider removing requirement of setuptools in pycryptopp's setup.py

[zooko]

It would be nice if you could build pycryptopp without having setuptools installed. There are two ways to achieve that:

a) don't require setuptools (although if setuptools is already present and if there is something that we could do to help users who have it, such as declaring extras dependencies or something, then we could continue to do that while still losing our _requirement_ for setuptools).

b) bundle (a fork of) setuptools and use that instead of importing the system's setuptools.

Now, we apparently used to do b), judging from the fact that a fork of setuptools is included in the pycryptopp source: git/misc/dependencies, but we don't currently use it in source:setup.py​ .

I would like to try strategy a) first and see what, if anything, we lose by removing our requirement on setuptools in setup.py.