Parent block header blob values not accessed correctly

### Metadata

* **Hardfork**: Prague (and maybe Cancun)

### What was wrong?

The following snippet using an `isinstance(..., Header)` check. This will behave incorrectly at the transition block.

https://github.com/ethereum/execution-specs/blob/f310a20b85126b3896056a818030f0fe72d6cd3c/src/ethereum/cancun/vm/gas.py#L288-L295

For example, consider what happens at the Prague transition block. The last Cancun block **has** the `excess_blob_gas` and `blob_gas_used` fields, but because `isinstance(parent_header, Header) == False` (Cancun headers are not Prague headers), the default values of `0` are used instead.