When signing a Taproot input (in a PSBT) using a key path spend we
currently return the pubkey associated with key that signs. However it
is common to think of the internal key as being the one that signs even
though this is not technically true. We also have the internal key in
the PSBT so matching against it is less surprising.
When using the `Psbt` type to sign a Taproot input using a key path
spend return the internal key.
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