gh-106320: Remove private _PyDict C API

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Move private _PyDict functions to the internal C API (pycore_dict.h):

• _PyDict_Contains_KnownHash()
• _PyDict_DebugMallocStats()
• _PyDict_DelItemIf()
• _PyDict_GetItemWithError()
• _PyDict_HasOnlyStringKeys()
• _PyDict_MaybeUntrack()
• _PyDict_MergeEx()

No longer export these functions.

• Issue: C API: Remove private C API functions (move them to the internal C API) #106320

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For now, I kept _PyDict_GetItemStringWithError() since it's used by _testcapi. The new public PyDict_GetItemStringRef() can be used by _testcapi instead.

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@methane: Do you want to prefer to keep a few of these private _PyDict functions for now? Or is it ok to remove them right now from the C API?

If one of these private functions make sense outside CPython code base, we should consider making it public: see https://discuss.python.org/t/c-api-my-plan-to-clarify-private-vs-public-functions-in-python-3-13/30131

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With this PR, there are still the following private _PyDict functions:

• _PyDictView_Intersect()
• _PyDictView_New()
• _PyDict_ContainsId()
• _PyDict_DelItemId()
• _PyDict_GetItemIdWithError()
• _PyDict_GetItemStringWithError()
• _PyDict_Next()
• _PyDict_Pop()
• _PyDict_SetItemId()
• _PyDict_SizeOf()

I consider removing them later, except of the "Id" variants.

[methane]

Cython uses _PyDict_GetItem_KnownHash and _PyDict_NewPresized().
So removing them will break some packages.
I don't know other private API users.

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The main branch of Cython uses the following private _PyDict functions:

• _PyDict_GetItem_KnownHash()
• _PyDict_NewPresized()
• _PyDict_Pop()
• _PyDict_SetItem_KnownHash()

Ok, let's keep these one for them, until we decide if we should make them public, or if Cython should avoid them.

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I updated my PR to keep the following functions:

• _PyDict_DelItem_KnownHash()
• _PyDict_GetItem_KnownHash()
• _PyDict_NewPresized()
• _PyDict_SetItem_KnownHash()

I also rebased it on the main branch.