Remove pin on mamba

[jtpio]

#121 added a pin on mamba <2:

xeus/environment-dev.yaml

Line 12 in 58b4a57

- mamba <2

To be able to continue importing from mamba with:

xeus/jupyterlite_xeus/create_conda_env.py

Line 10 in 4f43511

from mamba.api import create as mamba_create

We should look into eventually removing that pin.

cc @JohanMabille for awareness

[martinRenou]

We should actually remove mamba, see #148

Closing as wont fix