Switch immutables.Map and pyrsistent.PMap to constantdict #3076

Job	Run time
Documentation	2m 4s
Tests for downstream project Firedrake	15m 44s
Tests for downstream project arraycontext	2m 17s
Conda Examples	2m 3s
Tests for downstream project meshmode	4m 20s
Flake8	24s
Mypy	1m 54s
Tests for downstream project grudge	3m 59s
Pylint	2m 4s
Conda Pytest (ubuntu-latest)	2m 4s
Conda Pytest with Intel CL	2m 19s
Conda Pytest (macos-latest)	2m 20s
Conda Pytest without arg check	2m 13s
Tests for downstream project pytential	3m 37s
Tests for downstream project pytato	2m 37s
Conda Pytest Twice (for cache behavior)	2m 9s
Validate CITATION.cff	7s
	52m 15s

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