Error RE "weight" on master when using within Glyphs

Maybe I'm approaching this the wrong way, but when trying to use `glyphsLib` within Glyphs 3 itself:

```
import glyphsLib
font = Glyphs.font
ufos = glyphsLib.to_ufos(font)
```

I get:

```
Traceback (most recent call last):
File "", line 5
File "__init__.py", line 67, in to_ufos
result = list(builder.masters)
File "builders.py", line 200, in masters
self.to_ufo_font_attributes(self.family_name) # .font
File "font.py", line 40, in to_ufo_font_attributes
self.to_ufo_names(ufo, master, family_name) # .names
File "names.py", line 23, in to_ufo_names
to_ufo_names_roundtrip(master, ufo)
File "names.py", line 48, in to_ufo_names_roundtrip
weight = master.weight
AttributeError: 'NSKVONotifying_GSFontMaster' object has no attribute 'weight'
```

I wonder if this is a problem with `glyphsLib` thinking in terms of Glyphs 2 with a version 3 font. In Glyphs 2, `GSFontMaster.weight` will be defined, but in Glyphs 3, there is an array of axes in `GSFont.axes` where weight will be referenced - weight is not special and doesnt exist on the master. So maybe it is checking for it. But I'm not sure.

Start in names.py at to_ufo_names_roundtrip, where the traceback reaches master.weight, and inspect how to_ufos processes Glyphs 3 masters and axes. Reproduce the conversion with the provided Glyphs.font example; done means a Glyphs 3 font can reach UFO output without the AttributeError.

Written by the indexing model from the issue text.