Add py-multihash as dev dependency for tests

Fix test import error: 'ModuleNotFoundError: No module named multihash'

**Issue:**
cfxdb/tests/_util.py imports multihash for IPFS hash generation in
test utilities, but py-multihash wasn't declared as a dependency.

**Fix:**
Added py-multihash>=2.0.1 to dev optional dependencies in pyproject.toml

**Why dev dependency:**
- Only used in cfxdb/tests/_util.py for test data generation
- Not required for production use of cfxdb
- CI already installs dev dependencies via 'pip install -e .[dev]'

**Package info:**
- PyPI package: py-multihash (import as 'multihash')
- Latest version: 2.0.1
- Purpose: Multihash implementation for IPFS hash encoding
- Small, unproblematic dependency

Author: oberstet

pyproject.toml

@@ -68,6 +68,7 @@ dev = [
     "coverage>=5.0.0",
     "tox>=2.9.1",
     "tox-gh-actions>=2.2.0",
+    "py-multihash>=2.0.1",  # For IPFS hash generation in tests
 
     # Code quality
     "ruff>=0.1.0",  # Fast Python linter and formatter