bpo-33479: Remove unqualified tkinter threadsafe claim.

[terryjreedy]

It has not been true for several years and likely never was.
An explanation of what is true may be added later, after discussion,
and possibly after patching, but not before the coming 3.7.0rc1 release.

https://bugs.python.org/issue33479

[taleinat]

I agree that we should make this change now, and further consider additional clarification as per the discussion on the issue.

[terryjreedy]

Serhiy, unless you object, I plan to merge this in a day or two after the tests pass.
The outputs for VSTS: Linux and Windows just say "X Build not found"

[serhiy-storchaka]

The true is that _tkinter provides a threadsafe mechanism which allows Python and Tcl to interact. Its code could be simpler without it. If it doesn't always work, this can be classified as a bug and should be fixed. The fact that Python contains bugs should not mean that we should remove any claims about intended behavior.

[terryjreedy]

Requesting a review from Steve Dower was an accident. I was going to ask him about removing the failing VSTS tests but then discovered that they no longer matter.

[terryjreedy]

The tkinter thread code for threaded tcl works. The code for non-threaded tcl does not. See bpo-33257. I strongly feel that we should stop deceiving users with the latter into wasting time making futile attempts destined to fail. As my initial comment implies, this stripped down PR was only intended to be a minimal stop-gap, pending agreement on a replacement.

My initial post on the bpo issue proposes a proposed replacement sentence and a new section for the tkinter doc that say the minimum that I think users should know, including how to identify which tcl build they have. I would prefer to include the full proposal in this PR. If you approve of it, or are at least willing to discuss it, I will add it.

[vstinner]

I removed the " needs backport to 3.6" label, the 3.6 branch no longer accept bugfixes (only security fixes are accepted): https://devguide.python.org/#status-of-python-branches

[matrixise]

@terryjreedy Could I merge this one? Thank you

[miss-islington]

Thanks @terryjreedy for the PR, and @ambv for merging it 🌮🎉.. I'm working now to backport this PR to: 3.9, 3.10.
🐍🍒⛏🤖 I'm not a witch! I'm not a witch!

[bedevere-bot]

GH-27704 is a backport of this pull request to the 3.10 branch.

[bedevere-bot]

GH-27705 is a backport of this pull request to the 3.9 branch.