gitattribute: Mark generated files (GH-7619)

Marked files are collapsed by default in Github pull request.
https://github.com/github/linguist#generated-code

.gitattributes

@@ -37,3 +37,20 @@ Lib/test/test_importlib/data01/* -text
 *.proj text eol=crlf
 PCbuild/readme.txt text eol=crlf
 PC/readme.txt text eol=crlf
+
+# Generated files
+# https://github.com/github/linguist#generated-code
+Include/graminit.h          linguist-generated=true
+Python/graminit.h           linguist-generated=true
+Modules/clinic/*.h          linguist-generated=true
+Objects/clinic/*.h          linguist-generated=true
+PC/clinic/*.h               linguist-generated=true
+Python/clinic/*.h           linguist-generated=true
+Python/importlib.h          linguist-generated=true
+Python/importlib_external.h linguist-generated=true
+Include/Python-ast.h        linguist-generated=true
+Python/Python-ast.c         linguist-generated=true
+Include/opcode.h            linguist-generated=true
+Python/opcode_targets.h     linguist-generated=true
+Objects/typeslots.inc       linguist-generated=true
+Modules/unicodedata_db.h    linguist-generated=true