Add *.py diff=python to .gitattributes

[MeGaGiGaGon]

This adds the line *.py text diff=python to .gitattributes. I'm not 100% certain on all the effects this has, but it makes function name based finding git commands options like git log -L :<function_name>:<file_path> or git blame -L :<function_name> <file_path> work out of the box, which is a nice convenience.

[JelleZijlstra]

This appears to break test, something with CRLF last names.

diff --git a/tests/data/cases/preview_long_strings__east_asian_width.py b/tests/data/cases/preview_long_strings__east_asian_width.py

[MeGaGiGaGon]

As far as I can tell, the issue was because windows. I don't understand why anything broke, since github should use core.autocrlf=true by default, which should have the same behavior as text, but it doesn't. Removing the text still lets the diff function names work, but fixes the line ending nonsense to still be the core.autocrlf behavior.