Extend `impl FromPyObject<'a> for Cow<'a, [u8]>` to handle buffer protocol

[adamreichold]

In #2899, we temporarily introduced support for handling any Python object implementing the buffer protocol via impl FromPyObject<'a> for Cow<'a, [u8]> in 228cfd0.

We decided to remove this as it would only be available in CPython 3.11 or later or when the full API is enabled. When we drop support for CPython 3.10 or earlier, we should look into re-enabling this unconditionally.