Release notes for cobrapy 0.20.0

New features

There is a new function inspired by cameo that can load models directly from BiGG or BioModels and by default will cache them locally (#995), for example,
```
from cobra.io import load_model

ecoli = load_model("iML1515")
ylipo = load_model("MODEL1604280017")
```

Fixes

Over a series of pull requests, @synchon has been modernizing parts of the codebase for Python 3.6+ (#993, #999, #1000, #1003)
Upgrade the dependency python-libsbml-experimental to 5.18.3