Add additional generated files to .gitattributes

[emilyemorehouse]

I discovered a few generated files that are missing from the .gitattributes file -- this patch adds them.

(For anyone who is unfamiliar, this spec in the .gitattributes file is used by GitHub to suppress diffs for generated files automatically unless they are requested)

[serhiy-storchaka]

These files are not entirely generated. They contain both generated and hand written code. I think they shouldn't be added in the .gitattributes file.

[emilyemorehouse]

@serhiy-storchaka Understood, they essentially re-write themselves so anything in the files listed that is not pulled from contants/keywords could be manually changed by a human.

Perhaps a note as to which parts should be touched or not, or an inline mark as generated for the contants/keywords sections?

[serhiy-storchaka]

This may be changed in future. See bpo-30455.

[serhiy-storchaka]

Lib/token.py was added together with other generated files in #10370. Since Lib/symbol.py and Lib/keyword.py are not entirely generated and are rarely changed, I think they should not be added. Inline comments already mark the generated part and contain instructions for regenerating them.