sslutil: don't load default certificates when they aren't relevant
Before, we would call SSLContext.load_default_certs() when
certificate verification wasn't being used. Since
SSLContext.verify_mode == ssl.CERT_NONE, this would ideally
no-op. However, there is a slim chance the loading of system
certs could cause a failure. Furthermore, this behavior
interfered with a future patch that aims to provide a more
helpful error message when we're unable to load CAs.
The lack of test fallout is hopefully a sign that our
security code and tests are in a relatively good state.