refactor(dtype): automatically cast mixed string and date columns to strings #536

Job	Run time
check-docs	1m 15s
check-wheel-build (3.8, ubuntu-latest, x86-64)	1m 31s
test (3.8, ubuntu-latest)	1m 35s
lint	49s
check-wheel-build (3.8, ubuntu-latest, aarch64)	1m 54s
test (3.8, macos-latest)	2m 30s
check-wheel-build (3.8, macos-latest, x86-64)	1m 12s
check-wheel-build (3.8, macos-latest, aarch64)	54s
test (3.8, windows-latest)	4m 32s
check-wheel-build (3.8, windows-latest, x86-64)	3m 3s
test (3.9, ubuntu-latest)	1m 35s
test (3.9, macos-latest)	1m 32s
test (3.9, windows-latest)	3m 55s
test (3.10, ubuntu-latest)	1m 24s
test (3.10, macos-latest)	1m 37s
test (3.10, windows-latest)	3m 28s
test (3.11, ubuntu-latest)	1m 23s
test (3.11, macos-latest)	1m 8s
test (3.11, windows-latest)	3m 28s
test (3.12, ubuntu-latest)	1m 37s
test (3.12, macos-latest)	1m 19s
test (3.12, windows-latest)	4m 33s
	46m 14s

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