Move cryptography-based imports into fixture

Move the imports that require cryptography into the cert_factory
fixture.  This makes it possible to skip these tests on systems where
cryptography cannot be installed due to its dependency on Rust.

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 +550,8 @@ def deprecated_python() -> bool:
 
 @pytest.fixture(scope="session")
 def cert_factory(tmpdir_factory: pytest.TempdirFactory) -> CertFactory:
+    from tests.lib.certs import make_tls_cert, serialize_cert, serialize_key
+
     def factory() -> str:
         """Returns path to cert/key file."""
         output_path = Path(str(tmpdir_factory.mktemp("certs"))) / "cert.pem"