Use PIP_CONSTRAINT to fix 20.04 build

[carlcsaposs-canonical]

Context: canonical/charmcraft#2259

[paulomach]

I am still negotiating to drop it (Plan A).

[sinclert-canonical]

Thanks for looking into it. Judging by the history of failed CI runs, it took a while to figure out how to properly specify the constraints.txt file.

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Overall, I would rather drop support for Ubuntu 20.04. First, because it is more likely to bite us due to another compatibility problem in the future (as it has done now), and second because I cannot see any Canonical repositories with such a fine-tuned constraints.txt (see GitHub search), which tells me that we will be "alone" when it comes to evolve / tweak such a file.

This work-around will only be applicable if we cannot drop Ubuntu 20.04 support, and the migration to uv continues to be blocked.

[paulomach]

@carlcsaposs-canonical we can close this after got merged, or there`s anything else to it?

[carlcsaposs-canonical]

closing. realized we missed a couple things in #250, opened #259