feat: add `Form.is_equal_to` #1852

Job	Run time
Linux (3.12, -r requirements-test.txt)	2m 14s
Linux (3.11, -r requirements-test.txt)	4m 8s
Linux (3.10, -r requirements-test.txt)	4m 1s
Linux (3.9, -r requirements-test.txt)	4m 1s
Linux (3.8, -r requirements-test.txt)	3m 58s
Linux (3.8, -r requirements-test-minimal.txt)	1m 44s
Linux-cppyy	39s
Linux-ROOT-dask-awkward	3m 54s
MacOS (3.12)	2m 11s
MacOS (3.11)	4m 22s
MacOS (3.10)	6m 44s
MacOS (3.9)	5m 13s
MacOS (3.8)	4m 58s
Windows (3.12, x64)	2m 1s
Windows (3.11, x64)	3m 15s
Windows (3.10, x64)	3m 57s
Windows (3.9, x64)	3m 8s
Windows (3.8, x64)	4m 5s
Windows (3.9, x86)	6m 33s
	1h 11m 6s

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