Add hard assert on TreeSequence time units #911

Job	Run time
Build source distribution	17s
Build macOS/x86 wheels (3.8)	6m 6s
Build macOS/x86 wheels (3.9)	7m 30s
Build macOS/x86 wheels (3.10)	6m 55s
Build and test Linux wheels (python3.8)	9m 27s
Build macOS/x86 wheels (3.11)	6m 53s
Build and test Linux wheels (python3.9)	9m 28s
Build macOS/x86 wheels (3.12)	6m 39s
Build and test Linux wheels (python3.10)	8m 53s
Build and test Linux wheels (python3.11)	9m 1s
Build and test Linux wheels (python3.12)	9m 50s
Build package from source dist (3.8)	3m 29s
Build package from source dist (3.9)	3m 40s
Build package from source dist (3.10)	3m 30s
Build package from source dist (3.11)	3m 22s
Build package from source dist (3.12)	3m 50s
Test macOS/x86 wheels (3.8)	39s
Test macOS/x86 wheels (3.9)	42s
Test macOS/x86 wheels (3.10)	35s
Test macOS/x86 wheels (3.11)	56s
Test macOS/x86 wheels (3.12)	1m 7s
Upload to PyPI	19s
	1h 43m 8s

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