Rewrite setup/teardown methods as pytest fixtures, encapsulating concepts for clarity. Incidentally, this also fixes #231.

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.