python@3.8: use our own tcl-tk

[jamiesnape]

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Analog of #67378 and for the same reasons.

[carlocab]

Thanks, @jamiesnape.

Marking this as do not merge for the moment, as #68528 should probably be merged before this one. That's been open for a while and requires a rather long CI run, so I'd rather this PR was rebased to incorporate those changes than vice-versa.

[carlocab]

Rebased to incorporate changes from #68528.

@jamiesnape, please double-check that I resolved the merge conflicts correctly. Thanks!

[carlocab]

Thanks, @jamiesnape.

[BrewTestBot]

@carlocab has triggered a merge.