Casts columns to string before checking for special characters #187

Job	Run time
test-with-cov (3.11)	24s
Build and deploy docs to Pages	26s
Tests on Linux (3.7)	25s
Tests on Windows (3.6)	2m 4s
Tests on Linux (3.8)	21s
Tests on Windows (3.7)	1m 30s
Tests on Linux (3.9)	23s
Tests on Windows (3.8)	1m 50s
Tests on Linux (3.10)	28s
Tests on Windows (3.9)	1m 14s
Tests on Linux (3.11)	27s
Tests on Windows (3.10)	1m 16s
Tests on Windows (3.11)	1m 10s
Publish to PyPi	0s
	11m 58s

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