Some comments:
- since there is already a weekday() method, the new method should be
called yearday() rather than yday()
- ditto for fromyday(): fromyearday() would be better
- Modules/datetimemodule.c should only be indented with tabs, but your
patch indents it with spaces in some places
- the algorithm in date_fromyday() looks suboptimal: if you repeatedly
call days_in_month(), you shouldn't have to call days_before_month() at
the end, you can compute it by yourself

I haven't tested the patch yet.

Marc-André, do you have any take on the principle of this?