Fix #3330 by changing weight into string

The native part of the backend is actually checking whether the weight is `"bold"` (and the rest is normal) (see [_macosx.m:setfont] (https://github.com/matplotlib/matplotlib/blob/master/src/_macosx.m#L2345-L2362)).
So I'm checking `weight` against 700 and I'm putting bolder weights to `"bold"` (according to [`weight_dict`](https://github.com/matplotlib/matplotlib/blob/master/lib/matplotlib/font_manager.py#L91-L105)).