Skip cryptography-based tests when it is unavailable

Since the new versions of cryptography require Rust, it is no longer
possible to reliably install them on systems missing Rust support.
Given that only small subset of tests require certificates, skip them
when cryptography is unavailable.

tests/conftest.py

@@ -27,7 +27,6 @@
 from pip._internal.locations import _USE_SYSCONFIG
 from pip._internal.utils.temp_dir import global_tempdir_manager
 from tests.lib import DATA_DIR, SRC_DIR, PipTestEnvironment, TestData
-from tests.lib.certs import make_tls_cert, serialize_cert, serialize_key
 from tests.lib.path import Path
 from tests.lib.server import MockServer as _MockServer
 from tests.lib.server import make_mock_server, server_running
@@ -551,6 +550,11 @@ def deprecated_python() -> bool:
 
 @pytest.fixture(scope="session")
 def cert_factory(tmpdir_factory: pytest.TempdirFactory) -> CertFactory:
+    try:
+        from tests.lib.certs import make_tls_cert, serialize_cert, serialize_key
+    except ImportError:
+        pytest.skip("Cryptography is not available")
+
     def factory() -> str:
         """Returns path to cert/key file."""
         output_path = Path(str(tmpdir_factory.mktemp("certs"))) / "cert.pem"