.gitattributes: add a diff driver for Python

Declare that the *.py files in our tree are Python for the purposes of
diffing, and as in 00ddc9d ("Fix build with core.autocrlf=true",
2017-05-09) set eol=lf on them, which makes sense like with the *.perl
files.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

.gitattributes

@@ -4,6 +4,7 @@
 *.perl eol=lf diff=perl
 *.pl eof=lf diff=perl
 *.pm eol=lf diff=perl
+*.py eol=lf diff=python
 /Documentation/git-*.txt eol=lf
 /command-list.txt eol=lf
 /GIT-VERSION-GEN eol=lf