Load system default CAs when explicit CA not provided

[iAnomaly]

Resolves #1795.

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This change is 

[jeffwidman]

Thanks!

I noticed that #1795 limited loading the default to only when ssl_check_hostname was True.

It makes sense to me to fall back to defaults, but I have not worked a lot with SSL, so did you already consider this? Just want to make sure it was an intentional choice and not an oversight.

[iAnomaly]

@jeffwidman Thanks for confirming, but yes; it was an intentional choice:

Even if the user decides to ignore the hostname check against the certificate, the handshake will still fail without a valid chain of trust if the root certificate authority is unknown (i.e. without loading the system default CAs that contain the chains of trust).

[iAnomaly]

@jeffwidman I need this fix released to resolve a downstream consumer. Is it possible for you or another maintainer to cut a patch release?

[iAnomaly]

@dpkp Can you help here? Is it possible to cut a patch release with this fix? Thanks.

[dpkp]

Included in 1.4.7, which is now released + live on pypi