Without this change, a test failure will be displayed,
but not recognized as error by travis or other tools.

And execute unittest in verbose and buffered mode.

Added tests for different ciphers and different TLS Protocol versions.
Unfortenatlty, the TLS Protocol setting "ssl.PROTOCOL_TLS",
which is the Python ssl module default
and with which the ssl modul should choose the best matching protocol version
fails for the sslpsk client side.
Therefore we mark the test using this as "expected failure".

To improve readablity of the test results,
Python warnings will not be displayed (warnings="ignored").
Also test should best be started in buffered mode ("-b" on command line).

Code is formated by the tool "black".

Test multiple identities with different PSKs.

Also switched default TLS protocol
from
ssl.PROTOCOL_TLSv1
to
ssl.PROTOCOL_TLSv1_2
as this also works with Ubuntu 20.04.

The Bareos backup software also uses TLS-PSK
and offers a Python module to access the backup server.
As their identity uses a special format (TEXT + recordseperator (0x1E) + TEXT)
this test verifies, that this format does not cause problems inside sslpsk.

Test the client and server part of sslpsk independently.
This makes it easier to find potential problems.
The corresponding part is handled by the openssl binary
("openssl s_client" resp. "openssl s_server").

Change default TLS protocol from ssl.PROTOCOL_TLSv1_2
to ssl.PROTOCOL_TLS with fallback to ssl.PROTOCOL_SSLv23.
On Mac osx stick to ssl.PROTOCOL_TLSv1_2,
as ssl.PROTOCOL_TLS and ssl.PROTOCOL_SSLv23 are known to fail
in the Travis.org Mac osx test environment.

Also skip tests, if the required TLS protocol is not available.

This protocol version is deprecated since Python >= 3.6.
Instead ssl.PROTOCOL_TLS should be used.
However, some platforms in our test environment are to old to support ssl.PROTOCOL_TLS.