(Merge 3.1) Issue #12012: ssl.PROTOCOL_SSLv2 becomes optional

OpenSSL is now compiled with OPENSSL_NO_SSL2 defined (without the SSLv2
protocol) on Debian: fix the ssl module on Debian Testing and Debian Sid.

Optimize also ssl.get_protocol_name(): speed does matter!
diff --git a/Doc/library/ssl.rst b/Doc/library/ssl.rst
index a7e78bd..f4cac3a 100644
--- a/Doc/library/ssl.rst
+++ b/Doc/library/ssl.rst
@@ -237,6 +237,9 @@
 
    Selects SSL version 2 as the channel encryption protocol.
 
+   This protocol is not available if OpenSSL is compiled with OPENSSL_NO_SSL2
+   flag.
+
    .. warning::
 
       SSL version 2 is insecure.  Its use is highly discouraged.