chore: remove pyrfc3339 and change to datetime.datetime.fromisoformat… #1350

Job	Run time
Build test (3.10)	20s
Validate (3.10)	22s
Unit tests (3.8)	30s
Unit tests (3.9)	27s
Unit tests (3.10)	29s
Unit tests (3.11)	20s
Unit tests (3.12)	24s
Unit tests (3.13)	40s
Quarantined Integration Tests (3.10, 3.1/stable)	42m 38s
Integration (3.10, 3.1/stable, True)	40m 24s
Quarantined Integration Tests (3.10, 3.3/stable)	38m 49s
Integration (3.10, 3.1/stable, False)	39m 57s
Quarantined Integration Tests (3.10, 3.4/stable)	46m 37s
Integration (3.10, 3.3/stable, True)	37m 58s
Quarantined Integration Tests (3.10, 3.5/stable)	46m 11s
Integration (3.10, 3.3/stable, False)	37m 40s
Quarantined Integration Tests (3.10, 3.6/stable)	45m 47s
Integration (3.10, 3.4/stable, True)	38m 17s
Integration (3.10, 3.4/stable, False)	37m 57s
Integration (3.10, 3.5/stable, True)	37m 27s
Integration (3.10, 3.5/stable, False)	38m 27s
Integration (3.10, 3.6/stable, True)	38m 6s
Integration (3.10, 3.6/stable, False)	38m 11s
	10h 7m 58s

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