ext::sp_core::Pair is just a re export of the sp_core one, so I suspect you're importing a version of sp_keyring yourself that pulls in a different version of sp_core and trying to use the AccountKeyring from that?
Your options at the moment are to make sure you're using sp_keyring 7.0.0, or to do something like manually convert the Pair you get back into the Pair that sp_core 7.0.0 exposes and then pass that in to PairSigner.
There are issues updating Subxt to the latest substrate versions at the moment as a change there broke WASM compat, but I am working on it. That said, now that substrate crates are updating weekly, you may more frequently bump into this sort of version mismatch issue if you keep up to date with the latest substrate crates every week and Subxt has fallen behind.