use constantdict instead of immutabledict #1998

Job	Run time
Ruff	7s
Examples Conda Py3	1m 47s
Typos	5s
Pytest Conda Py3 POCL	2m 58s
Pytest Conda Py3 Intel	2m 31s
Tests for downstream project mirgecom	4s
Tests for downstream project grudge	12m 3s
Documentation	2m 33s
Tests for downstream project meshmode	10m 28s
Tests for downstream project mirgecom_examples	4s
basedpyright	2m 13s
	34m 53s

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