Bug#974991: sagemath: segfault on startup

[Sylvain LÉVÊQUE]

Hello

>From trial and error, my workaround is to downgrade python3-cypari2. I
have no idea whether it is appropriate or if it is going to trigger
something else, but at least I don't have a startup crash anymore.
Here are the steps:
- add "deb https://snapshot.debian.org/archive/debian/20200628T024451Z
unstable main" to /etc/apt/sources.list
- sudo apt -o Acquire::Check-Valid-Until=false update
- sudo apt install python3-cypari2=2.1.1-2+b2

It seems it is a recurring situation, 906796 happened two years ago.
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Sylvain