Python 3.9.7 breaks tests

[aaugustin]

Tests fail due to https://bugs.python.org/issue45097. It will be fixed in 3.9.8. We could pin GitHub actions to 3.9.6 (and inevitably forget to unpin...) or just live with broken builds for two months, until GitHub actions gives us 3.9.8.

[aaugustin]

Revert around mid november 2021, when 3.9.8 is available on GitHub actions.

[vfazio]

3.9.8 has been released https://www.python.org/downloads/release/python-398/

[aaugustin]

Thank you.