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

[3.15] gh-154053: Fix compilation of the ssl module against LibreSSL (GH-154054) (GH-154075)

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.
(cherry picked from commit 39b058f8ad29238edc8b65406bb1fb0bc3c0020d)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>