onnx 1.13.0 does not work with protobuf <=3.20.1

[hagenw]

When trying to install the packages we get:

ERROR: pip's dependency resolver does not currently take into account all the packages that are installed. This behaviour is the source of the following dependency conflicts.
399
onnx 1.13.0 requires protobuf<4,>=3.20.2, but you have protobuf 3.20.1 which is incompatible.

The problem is that we specify

protobuf <=3.20.1  # avoid TypeError: Descriptors cannot not be created directly

in docs/requirements.txt.
We need to check how to make this work for newer versions of protobuf, for now I will restrict onnx to <1.13.0 in docs/requriements.txt in #47.