importlib_metadata is a dependency on old Python versions (#2484)

Fixing a related error on the conda-forge update for maturin builds
conda-forge/sourmash-minimal-feedstock#40
where `importlib_metadata` is missing due to not being declared as dependency
for old Python versions (pre-3.10)

pyproject.toml

@@ -78,6 +78,7 @@ dependencies = [
   "deprecation>=2.0.6",
   "cachetools>=4,<6",
   "bitstring>=3.1.9,<5",
+  "importlib_metadata;python_version<'3.10'"
 ]
 
 requires-python = ">=3.8"