bpo-18233: Add internal methods to access peer chain (GH-25467)

The internal `_ssl._SSLSocket` object now provides methods to retrieve
the peer cert chain and verified cert chain as a list of Certificate
objects. Certificate objects have methods to convert the cert to a dict,
PEM, or DER (ASN.1).

These are private APIs for now. There is a slim chance to stabilize the
approach and provide a public API for 3.10. Otherwise I'll provide a
stable API in 3.11.

Signed-off-by: Christian Heimes <christian@python.org>