[3.10] gh-123678: Upgrade libexpat 2.6.3 #124411

Job	Run time
Check for source changes	8s
Check for source changes	8s
Check if generated files are up to date	4m 16s
Check if generated files are up to date	4m 16s
Windows (x86)	9m 51s
Windows (x86)	9m 51s
Windows (x64)	10m 3s
Windows (x64)	10m 3s
macOS	12m 21s
macOS	10m 47s
Ubuntu	7m 59s
Ubuntu	7m 59s
Check if the ABI has changed	1m 15s
Check if the ABI has changed	1m 15s
Ubuntu SSL tests with OpenSSL	0s
Ubuntu SSL tests with OpenSSL	0s
	1h 30m 12s

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