Use absolute instead of relative imports for vendored modules

Co-authored-by: Tzu-ping Chung <uranusjr@gmail.com>

src/pip/_internal/cli/req_command.py

@@ -58,7 +58,7 @@ def _create_truststore_ssl_context() -> Optional["SSLContext"]:
         return None
 
     try:
-        from ..._vendor import truststore
+        from pip._vendor import truststore
     except ImportError:
         raise CommandError(
             "To use the truststore feature, 'truststore' must be installed into "