Add .gitattributes file for better behavior on Windows-based systems #439

Job	Run time
build-macos-openssl-1-0-2 (macos-latest)	2m 26s
build-macos (1.1)	40s
build-ubuntu (ubuntu-latest)	15m 14s
build-notls	30s
build-macos (3.0)	37s
build-ubuntu (ubuntu-20.04)	3m 11s
	22m 38s

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