Python 3.10 migration for pillow stalled in 'not-solvable'

The Python 3.10 migration for `pillow` is stalled in 'not-solvable', with the following error message ([link](https://github.com/regro/autotick-bot/actions/runs/1429516176)):

```
linux_64_python3.10.____cpython: Encountered problems while solving:
- package python_abi-3.10-2_cp310 requires python 3.10.*, but none of the providers can be installed

linux_64_python3.7.____cpython: Encountered problems while solving:
- package requests-2.18.4-py27hc5b0589_1 requires python >=2.7,<2.8.0a0, but none of the providers can be installed

linux_64_python3.8.____cpython: Encountered problems while solving:
- package requests-2.18.4-py27hc5b0589_1 requires python >=2.7,<2.8.0a0, but none of the providers can be installed

linux_64_python3.9.____cpython: Encountered problems while solving:
- package requests-2.18.4-py27hc5b0589_1 requires python >=2.7,<2.8.0a0, but none of the providers can be installed

linux_aarch64_python3.10.____cpython: Encountered problems while solving:
- nothing provides openssl >=1.0.2p,<1.0.3a needed by python-3.7.1-hd75c258_1001

linux_aarch64_python3.7.____cpython: Encountered problems while solving:
- package requests-2.24.0-py_0 requires urllib3 >=1.21.1,<1.26,!=1.25.0,!=1.25.1, but none of the providers can be installed

linux_aarch64_python3.8.____cpython: Encountered problems while solving:
- nothing provides openssl >=1.0.2p,<1.0.3a needed by python-2.7.15-h938d71a_1006

linux_aarch64_python3.9.____cpython: Encountered problems while solving:
- nothing provides openssl >=1.0.2p,<1.0.3a needed by python-2.7.15-h938d71a_1006

linux_ppc64le_python3.10.____cpython: Encountered problems while solving:
- package requests-2.18.4-py27_1 requires python >=2.7,<2.8.0a0, but none of the providers can be installed

linux_ppc64le_python3.7.____cpython: Encountered problems while solving:
- package requests-2.18.4-py27_1 requires python >=2.7,<2.8.0a0, but none of the providers can be installed

linux_ppc64le_python3.8.____cpython: Encountered problems while solving:
- package requests-2.18.4-py27_1 requires python >=2.7,<2.8.0a0, but none of the providers can be installed

linux_ppc64le_python3.9.____cpython: Encountered problems while solving:
- package requests-2.18.4-py27_1 requires python >=2.7,<2.8.0a0, but none of the providers can be installed

osx_64_python3.10.____cpython: Encountered problems while solving:
- package requests-2.18.4-py27h9b2b37c_1 requires python >=2.7,<2.8.0a0, but none of the providers can be installed

osx_64_python3.7.____cpython: Encountered problems while solving:
- package requests-2.18.4-py27h9b2b37c_1 requires python >=2.7,<2.8.0a0, but none of the providers can be installed

osx_64_python3.8.____cpython: Encountered problems while solving:
- package requests-2.18.4-py27h9b2b37c_1 requires python >=2.7,<2.8.0a0, but none of the providers can be installed

osx_64_python3.9.____cpython: Encountered problems while solving:
- package requests-2.18.4-py27h9b2b37c_1 requires python >=2.7,<2.8.0a0, but none of the providers can be installed

osx_arm64_python3.10.____cpython: Encountered problems while solving:
- package requests-2.24.0-py_0 requires urllib3 >=1.21.1,<1.26,!=1.25.0,!=1.25.1, but none of the providers can be installed

osx_arm64_python3.8.____cpython: Encountered problems while solving:
- package requests-2.24.0-py_0 requires urllib3 >=1.21.1,<1.26,!=1.25.0,!=1.25.1, but none of the providers can be installed

osx_arm64_python3.9.____cpython: Encountered problems while solving:
- package requests-2.24.0-py_0 requires urllib3 >=1.21.1,<1.26,!=1.25.0,!=1.25.1, but none of the providers can be installed

win_64_python3.10.____cpython: Encountered problems while solving:
- nothing provides vc 10.* needed by openssl-1.0.2h-vc10_0

win_64_python3.7.____cpython: Encountered problems while solving:
- nothing provides vc 10.* needed by openssl-1.0.2h-vc10_0

win_64_python3.8.____cpython: Encountered problems while solving:
- nothing provides vc 10.* needed by openssl-1.0.2h-vc10_0

win_64_python3.9.____cpython: Encountered problems while solving:
- nothing provides vc 10.* needed by openssl-1.0.2h-vc10_0

```

If I manually apply the python310.yaml file and regenerate the feedstock, I can solve environments on my own machine using mambabuild. What is required to get this unstuck (mainly so that `matplotlib` can be migrated)?