GitLab

This commit adds:

1) Methods to SSLContext class that match CPython signature:

	- `SSLContext.load_cert_chain(certfile, keyfile)`
	- `SSLContext.load_verify_locations(cafile=, cadata=)`
	- `SSLContext.get_ciphers()` --> ["CIPHERSUITE"]
	- `SSLContext.set_ciphers(["CIPHERSUITE"])`

2) `sslsocket.cipher()` to get current ciphersuite and protocol
   version.

3) `ssl.MBEDTLS_VERSION` string constant.

4) Certificate verification errors info instead of
   `MBEDTLS_ERR_X509_CERT_VERIFY_FAILED`.

5) Tests in `net_inet` and `multi_net` to test these new methods.

`SSLContext.load_cert_chain` method allows loading key and cert from disk
passing a filepath in `certfile` or `keyfile` options.

`SSLContext.load_verify_locations`'s `cafile` option enables the same
functionality for ca files.
Signed-off-by: Carlos Gil <carlosgilglez@gmail.com>