Enable installation of aiida-core with conda and Python 3.9

[csadorf]

It is currently not possible to install aiida-core with Python 3.9, see minimal example:

$ conda create -n aiida-py39 python=3.9 aiida-core
Collecting package metadata (current_repodata.json): \done
Solving environment: failed with repodata from current_repodata.json, will retry with next repodata source.
Collecting package metadata (repodata.json): done
Solving environment: /
Found conflicts! Looking for incompatible packages.
This can take several minutes.  Press CTRL-C to abort.
failed

UnsatisfiableError: The following specifications were found to be incompatible with each other:

Output in format: Requested package -> Available versions

Package python conflicts for:
aiida-core -> aio-pika~=6.6 -> python[version='2.7.*|3.5.*|3.6.*|>=3.6|>=3.9,<3.10.0a0|>=3|>=3.5,<3.6.0a0|=2.7|>=3.5|>=2.7|3.4.*|3.8.*|3.7.*']
python=3.9
aiida-core -> python[version='>=2.7,<2.8.0a0|>=3.5|~=3.7|>=3.6,<3.7.0a0|>=3.8,<3.9.0a0|>=3.7,<3.8.0a0']

[csadorf]

@ltalirz Should we rather track this in the https://github.com/conda-forge/aiida-core-feedstock repository?

[ltalirz]

yes, I think so

[csadorf]

@ltalirz Can you transfer the issue ? I am lacking permissions to do so.

[ltalirz]

As far as I am aware, one can only transfer issues within the same github organisation. I.e. we will need to open a separate one in any case. I'll take care of it