GeezerVison

Since gh-138252 (TLS signature-algorithm support, 3.15+), _ssl fails to compile against LibreSSL, which does not provide SSL_CTX_set1_sigalgs_list() / SSL_CTX_set1_client_sigalgs_list():

Modules/_ssl.c:3867: call to undeclared function 'SSL_CTX_set1_client_sigalgs_list'
Modules/_ssl.c:3887: call to undeclared function 'SSL_CTX_set1_sigalgs_list'

With -Werror=implicit-function-declaration this is fatal, so _ssl is not built at all on LibreSSL (regression from 3.14).

This PR guards the set_client_sigalgs/set_server_sigalgs methods so they raise NotImplementedError on LibreSSL (matching how other library-unavailable features are handled in _ssl.c), and skips the corresponding test_ssl tests.

Verified:

• OpenSSL 3.5.5: builds clean, sigalgs tests pass (unaffected).
• LibreSSL 4.3.0 (OpenBSD 7.9): _ssl now compiles; the 7 sigalgs tests skip; the remaining test_ssl failures are identical to those already present on 3.14 (pre-existing LibreSSL divergences), i.e. no new failures.

• Issue: _ssl fails to compile against LibreSSL (3.15+): SSL_CTX_set1_sigalgs_list / SSL_CTX_set1_client_sigalgs_list undeclared #154053