Allow use of list[str] inside plugins

Fixes: #3287

docs/changelog/3288.bugfix.rst

@@ -0,0 +1 @@
+Allow ``ConfigSet.add_config`` to receive parameterized generics for ``of_type``.

src/tox/config/loader/convert.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import typing
 from abc import ABC, abstractmethod
 from collections import OrderedDict
 from pathlib import Path
@@ -39,6 +40,10 @@ def to(self, raw: T, of_type: type[V], factory: Factory[V]) -> V:  # noqa: PLR09
             return self.to_env_list(raw)  # type: ignore[return-value]
         if issubclass(of_type, str):
             return self.to_str(raw)  # type: ignore[return-value]
+        # python does not allow use of parametrized generics with isinstance,
+        # so we need to check for them.
+        if isinstance(raw, typing.GenericAlias) and typing.get_args(of_type) and typing.get_args(of_type)[0] is str:  # type: ignore[attr-defined]
+            return raw  # type: ignore[return-value]
         if isinstance(raw, of_type):  # already target type no need to transform it
             # do it this late to allow normalization - e.g. string strip
             return raw