fix: correct broken format string in decode() TypeError message

[AmSach]

Fixes #47.

What was broken

The decode() function in multihash/multihash.py raised a TypeError using a non-f-string format:

raise TypeError("multihash should be bytes, not {}", type(multihash))

Because the literal string is not prefixed with f, the {} placeholder is not interpolated — it is printed verbatim, with the offending type captured only as a separate element of e.args. The user sees:

TypeError: multihash should be bytes, not {}
# Expected: TypeError: multihash should be bytes, not <class 'str'>

Fix

Switch the literal to a proper f-string so the type is interpolated into the message at raise time:

raise TypeError(f"multihash should be bytes, not {type(multihash)}")

Tested

• Added test_decode_type_error_message_interpolates_type in tests/test_multihash.py that asserts the message contains "should be bytes, not" and does not contain the literal "{}".
• Verified the new test fails on the unpatched code (literal {} present) and passes on the patched code.
• Full test suite: 229 tests pass.
• Lint: ruff check clean on touched files.

Files changed

• multihash/multihash.py — fix the format string (1 line)
• tests/test_multihash.py — add regression test (21 lines)
• newsfragments/47.bugfix.rst — release note (new file)