Switch immutables.Map and pyrsistent.PMap to constantdict #3081

Job	Run time
Documentation	1m 57s
Tests for downstream project Firedrake	15m 33s
Tests for downstream project arraycontext	2m 39s
Conda Examples	2m 18s
Tests for downstream project meshmode	10m 50s
Flake8	25s
Mypy	2m 6s
Tests for downstream project grudge	9m 36s
Tests for downstream project pytential	33m 42s
Pylint	2m 28s
Conda Pytest (ubuntu-latest)	4m 15s
Conda Pytest with Intel CL	4m 27s
Tests for downstream project pytato	5m 27s
Conda Pytest (macos-latest)	7m 1s
Conda Pytest without arg check	4m 12s
Conda Pytest Twice (for cache behavior)	5m 16s
Validate CITATION.cff	7s
	1h 52m 19s

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