gh-154053: Fix compilation of the ssl module against LibreSSL (GH-154054)

gh-154053: Fix compilation of the ssl module against LibreSSL (GH-154054)

LibreSSL does not provide SSL_CTX_set1_sigalgs_list() or
SSL_CTX_set1_client_sigalgs_list(), added in gh-138252, so _ssl failed to
compile against LibreSSL on 3.15+. Guard the set_server_sigalgs() and
set_client_sigalgs() methods so they raise NotImplementedError on LibreSSL,
and skip the corresponding tests.