bpo-34271: Add ssl debugging helpers (GH-10031)

The ssl module now can dump key material to a keylog file and trace TLS
protocol messages with a tracing callback. The default and stdlib
contexts also support SSLKEYLOGFILE env var.

The msg_callback and related enums are private members. The feature
is designed for internal debugging and not for end users.

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