fix: Deactivate Pydantic validation on Python 3.9 is `eval-type-backp…

…ort` is not available (for modern typing syntax support)

Issue-241: #241

src/mkdocstrings_handlers/python/config.py

@@ -7,12 +7,18 @@
 from dataclasses import field, fields
 from typing import TYPE_CHECKING, Annotated, Any, Literal
 
+from mkdocstrings.loggers import get_logger
+
 # YORE: EOL 3.10: Replace block with line 2.
 if sys.version_info >= (3, 11):
     from typing import Self
 else:
     from typing_extensions import Self
 
+
+logger = get_logger(__name__)
+
+
 try:
     # When Pydantic is available, use it to validate options (done automatically).
     # Users can therefore opt into validation by installing Pydantic in development/CI.
@@ -30,6 +36,17 @@
     if getattr(pydantic, "__version__", "1.").startswith("1."):
         raise ImportError  # noqa: TRY301
 
+    if sys.version_info < (3, 10):
+        try:
+            import eval_type_backport  # noqa: F401
+        except ImportError:
+            logger.debug(
+                "Pydantic needs the `eval-type-backport` package to be installed "
+                "for modern type syntax to work on Python 3.9. "
+                "Deactivating Pydantic validation for Python handler options.",
+            )
+            raise
+
     from inspect import cleandoc
 
     from pydantic import Field as BaseField