Round-tripping single UFO: no axes defined in designspace

1. `git clone https://github.com/daltonmaag/ubuntu`
2. `ufo2glyphs ubuntu/source/Ubuntu-R.ufo`
3. `glyphs2ufo ubuntu/source/Ubuntu.glyphs`
4. `ufo2glyphs ubuntu/source/Ubuntu.designspace`
```
Traceback (most recent call last):
File "/Users/nikolaus/Entwicklung/glyphsLib/venv/bin/ufo2glyphs", line 11, in
load_entry_point('glyphsLib', 'console_scripts', 'ufo2glyphs')()
File "/Users/nikolaus/Entwicklung/glyphsLib/Lib/glyphsLib/cli.py", line 241, in _ufo2glyphs_entry_point
return main(args)
File "/Users/nikolaus/Entwicklung/glyphsLib/Lib/glyphsLib/cli.py", line 145, in main
return options.func(options)
File "/Users/nikolaus/Entwicklung/glyphsLib/Lib/glyphsLib/cli.py", line 197, in ufo2glyphs
designspace.read(designspace_file)
File "/Users/nikolaus/Entwicklung/glyphsLib/venv/lib/python3.7/site-packages/fontTools/designspaceLib/__init__.py", line 1031, in read
reader.read()
File "/Users/nikolaus/Entwicklung/glyphsLib/venv/lib/python3.7/site-packages/fontTools/designspaceLib/__init__.py", line 675, in read
self.readSources()
File "/Users/nikolaus/Entwicklung/glyphsLib/venv/lib/python3.7/site-packages/fontTools/designspaceLib/__init__.py", line 792, in readSources
sourceObject.location = self.locationFromElement(sourceElement)
File "/Users/nikolaus/Entwicklung/glyphsLib/venv/lib/python3.7/site-packages/fontTools/designspaceLib/__init__.py", line 824, in locationFromElement
elementLocation = self.readLocationElement(locationElement)
File "/Users/nikolaus/Entwicklung/glyphsLib/venv/lib/python3.7/site-packages/fontTools/designspaceLib/__init__.py", line 831, in readLocationElement
raise DesignSpaceDocumentError("No axes defined")
fontTools.designspaceLib.DesignSpaceDocumentError: No axes defined
```

Reproduce the four-step round trip with the Ubuntu repository, then start in Lib/glyphsLib/cli.py and follow the designspace read path shown in the traceback into fontTools designspaceLib. Done means the single-UFO round trip no longer fails with “No axes defined” and produces the expected output.

Written by the indexing model from the issue text.