gh-88965: typing: fix type substitution of a list of types after initial ParamSpec substitution (GH-102808)

Previously, this used to fail:

```py
from typing import *
T = TypeVar("T")
P = ParamSpec("P")

class X(Generic[P]):
    f: Callable[P, int]

Y = X[[int, T]]
Z = Y[str]
```

(cherry picked from commit adb0621652)

Co-authored-by: Nikita Sobolev <mail@sobolevn.me>
Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

Lib/typing.py

@@ -250,10 +250,17 @@ def _collect_parameters(args):
     """
     parameters = []
     for t in args:
-        # We don't want __parameters__ descriptor of a bare Python class.
         if isinstance(t, type):
-            continue
-        if hasattr(t, '__typing_subst__'):
+            # We don't want __parameters__ descriptor of a bare Python class.
+            pass
+        elif isinstance(t, tuple):
+            # `t` might be a tuple, when `ParamSpec` is substituted with
+            # `[T, int]`, or `[int, *Ts]`, etc.
+            for x in t:
+                for collected in _collect_parameters([x]):
+                    if collected not in parameters:
+                        parameters.append(collected)
+        elif hasattr(t, '__typing_subst__'):
             if t not in parameters:
                 parameters.append(t)
         else:
@@ -1416,10 +1423,12 @@ class _GenericAlias(_BaseGenericAlias, _root=True):
             raise TypeError(f"Too {'many' if alen > plen else 'few'} arguments for {self};"
                             f" actual {alen}, expected {plen}")
         new_arg_by_param = dict(zip(params, args))
+        return tuple(self._make_substitution(self.__args__, new_arg_by_param))
 
+    def _make_substitution(self, args, new_arg_by_param):
+        """Create a list of new type arguments."""
         new_args = []
-        for old_arg in self.__args__:
-
+        for old_arg in args:
             if isinstance(old_arg, type):
                 new_args.append(old_arg)
                 continue
@@ -1463,10 +1472,20 @@ class _GenericAlias(_BaseGenericAlias, _root=True):
                 # should join all these types together in a flat list
                 # `(float, int, str)` - so again, we should `extend`.
                 new_args.extend(new_arg)
+            elif isinstance(old_arg, tuple):
+                # Corner case:
+                #    P = ParamSpec('P')
+                #    T = TypeVar('T')
+                #    class Base(Generic[P]): ...
+                # Can be substituted like this:
+                #    X = Base[[int, T]]
+                # In this case, `old_arg` will be a tuple:
+                new_args.append(
+                    tuple(self._make_substitution(old_arg, new_arg_by_param)),
+                )
             else:
                 new_args.append(new_arg)
-
-        return tuple(new_args)
+        return new_args
 
     def copy_with(self, args):
         return self.__class__(self.__origin__, args, name=self._name, inst=self._inst,