Fix platformdirs dependency

[petertdavies]

What was wrong?

The platformdirs dependency is required by ethereum-spec-evm daemon, but is only depended on by the test extra.

How was it fixed?

Move platformdirs dependency to install-requires.

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[SamWilsn]

platformdirs is not required to use the specs as a library, so I think it should remain optional. Perhaps we do something like this instead:

execution-specs/src/ethereum_optimized/state_db.py

Lines 20 to 27 in fcd1275

	try:
	import rust_pyspec_glue
	except ImportError as e:
	# Add a message, but keep it an ImportError.
	raise e from Exception(
	"Install with `pip install 'ethereum[optimized]'` to enable this "
	"package"
	)