extmod/modssl: Add SSLContext class.

This commit adds the SSLContext class to the ssl module, and retains the
existing ssl.wrap_socket() function to maintain backwards compatibility.

CPython deprecated the ssl.wrap_socket() function since CPython 3.7 and
instead one should use ssl.SSLContext().wrap_socket().  This commit makes
that possible.

For the axtls implementation:
- ssl.SSLContext is added, although it doesn't hold much state because
  axtls requires calling ssl_ctx_new() for each new socket
- ssl.SSLContext.wrap_socket() is added
- ssl.PROTOCOL_TLS_CLIENT and ssl.PROTOCOL_TLS_SERVER are added

For the mbedtls implementation:
- ssl.SSLContext is added, and holds most of the mbedtls state
- ssl.verify_mode is added (getter and setter)
- ssl.SSLContext.wrap_socket() is added
- ssl.PROTOCOL_TLS_CLIENT and ssl.PROTOCOL_TLS_SERVER are added

The signatures match CPython:
- SSLContext(protocol)
- SSLContext.wrap_socket(sock, *, server_side=False,
    do_handshake_on_connect=True, server_hostname=None)

The existing ssl.wrap_socket() functions retain their existing signature.

Signed-off-by: Damien George <damien@micropython.org>

tests/extmod/ssl_basic.py

@@ -33,7 +33,7 @@ except OSError as er:
 ss = ssl.wrap_socket(TestSocket(), server_side=1, do_handshake=0)
 
 # print
-print(repr(ss)[:12])
+print(ss)
 
 # setblocking() propagates call to the underlying stream object
 ss.setblocking(False)