CI: bump Coq to 8.20 and mathcomp to 2.3.0

Adopt the latest Coq and mathcomp versions now that CoqEAL has been updated.

assigned to @bbb

added 1 commit

• c46131d5 - bump OPAM dependencies

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added 1 commit

• 7cc7f8df - CI: tell `pip` that it's ok to install packages in containers

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added 4 commits

• b9f30084 - 1 commit from branch master
• 290ca6aa - CI: bump Coq to 8.20 and mathcomp to 2.3.0
• 709d9091 - bump OPAM dependencies
• c2df643c - CI: tell `pip` that it's ok to install packages in containers

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merged