core: Put Python version as a project requirement so it is considered by ruff (#26608)

Ruff doesn't know about the python version in
`[tool.poetry.dependencies]`. It can get it from
`project.requires-python`.

Notes:
* poetry seems to have issues getting the python constraints from
`requires-python` and using `python` in per dependency constraints. So I
had to duplicate the info. I will open an issue on poetry.
* `inspect.isclass()` doesn't work correctly with `GenericAlias`
(`list[...]`, `dict[..., ...]`) on Python <3.11 so I added some `not
isinstance(type, GenericAlias)` checks:

Python 3.11
```pycon
>>> import inspect
>>> inspect.isclass(list)
True
>>> inspect.isclass(list[str])
False
```

Python 3.9
```pycon
>>> import inspect
>>> inspect.isclass(list)
True
>>> inspect.isclass(list[str])
True
```

Co-authored-by: Eugene Yurtsev <eyurtsev@gmail.com>

libs/core/tests/unit_tests/load/test_serializable.py

@@ -1,5 +1,3 @@
-from typing import Dict
-
 from pydantic import ConfigDict, Field
 
 from langchain_core.load import Serializable, dumpd, load
@@ -56,7 +54,7 @@ def test_simple_serialization_secret() -> None:
             return True
 
         @property
-        def lc_secrets(self) -> Dict[str, str]:
+        def lc_secrets(self) -> dict[str, str]:
             return {"secret": "MASKED_SECRET", "secret_2": "MASKED_SECRET_2"}
 
     foo = Foo(