Update python version to `3.9` about tribler HOT 6 OPEN

I have opted to use pyenv for setting up the python environment and updated the Jenkins script to use the correct python using pyenv local setup. So far, macOS and Linux builders are updated and the build seems to work too. But, Windows installation seems a bit challenging.

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For Windows, you might consider using Chocolatey as it may simplify things: Python 3.9 on Chocolatey.

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The reason why Python3.9 was not working on our builders is because Python3.9 is not supported on Win7. Therefore, the windows builders need to be upgraded. This will likely affect some users using Win7 still.

I'll upgrade the builders on Win10 machines.

Ref: https://bugs.python.org/issue41412

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Priority for upgrading the builders is put on hold until the existing bugs are fixed.
I'm unassigning myself till it becomes a priority again.

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@xoriole, "until the existing bugs are fixed" does sound a bit like "never"...

It seems like I should revert the changes I made to GitHub Actions because now it's possible that one of the developers will use a feature from Python 3.9, which will pass all the tests in a PR as they operate with Python 3.9. However, when we try to build the next release (for example, 7.14), we might encounter these incompatibilities since the builders use Python 3.8.

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@xoriole the issue you mentioned is fixed:

• python/cpython#85584

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