FreeBSD: Add root certificate from ca_root_nss.

On FreeBSD root certificates from certificate authorities included in the
Mozilla NSS library are provided by the ca_root_nss package:

jcigar@frodon:~/ > pkg info -l ca_root_nss-3.21
ca_root_nss-3.21:
    /usr/local/etc/ssl/cert.pem.sample
    /usr/local/openssl/cert.pem.sample
    /usr/local/share/certs/ca-root-nss.crt
    /usr/local/share/licenses/ca_root_nss-3.21/LICENSE
    /usr/local/share/licenses/ca_root_nss-3.21/MPL
    /usr/local/share/licenses/ca_root_nss-3.21/catalog.mk

On some machines there is no symbolic link (/etc/ssl/cert.pem) installed

Test on pypy3

Adapt resolution of classes from importlib.machinery. Restores compatibility for PyPy3 where importlib.machinery exists but FileFinder and SourceFileLoader aren't implemented.

Rather than re-use globals of setuptools.launch, build a unique namespace in which to launch the script. Prevents imports from occuring relative to 'setuptools' on Python 2. Ref #490.

Also hide Cython when hiding setuptools, as setuptools will have imported Cython and Cython references the distutils.Extension. Fixes #488.

Remove JythonCommandSpec as it's not required on Jython 2.7 and setuptools no longer supports anything 2.5, but had to test for 'Jython on Windows' in two other places

Relicense the package as MIT license. Drop licensing as PSF and Zope as neither of those licenses grant authority for use outside their respective projects. Vendored projects are licensed under their respective licenses, currently MIT for six and Apache v2 for packaging. Fixes #132.