Implement-get_ca_certs-for-macos

[BrokenDuck]

Rationale

On Mac OS, the load_default_certs SSLContext method does not load the system root certificate. In order for the get_ca_certs method to work properly on mac os, we need to override the load_defaults_certs method on Mac OS.

Method

We extract the certificates using the Security Framework, with the SecTrustCopyAnchorCertificates function. We then load the certificates using the load_verify_location SSLContext method which accepts DER-encoded byte-like certificates in the cadata argument.

[jmaillefaud]

Pinging @sethmlarson

[sethmlarson]

I'm not sure I understand the justification in the pull request description:

In order for the get_ca_certs method to work properly on mac os, we need to override the load_defaults_certs method on Mac OS.

I don't think this is correct? get_ca_certs() returns whatever certificates are loaded, in the case of Truststore it's whatever has been loaded manually plus the system certificates. We don't need load_default_certs() implemented for this to work.

Truststore on its own should behave as if load_default_certs() has already been called on all truststore.SSLContext() objects, as the default behavior is to use the system certificates.