Lib: re-word or remove outdated comments.

[jepler]

I searched across the Lib directory of CPython and removed or re-worded outdated comments.

For instance, the private _is_text_encoding was not in fact removed in Python 3.5 but is still used by the implementation.

The _clamp property of Decimal context objects actually was removed in Python 3.3, but a test for its removal (with outdated comment) still only speculated about its eventual removal.

[python-cla-bot]

All commit authors signed the Contributor License Agreement.

[terryjreedy]

My opinions
codecs: yes
idlelib: please revert for now
logging: maintainer should have last say, but OK by me. Does the handlers doc have 'new in 3.2' note?
readline: yes
async test: yes
decimal test: yes
wsgiref handlers: yes

[vsajip]

I'm fine with the change to logging. The versionadded is there in the docs for QueueHandler.

[jepler]

idlelib: please revert for now

I'm happy to do this if it makes it easier to get the rest in. But, can you say more about why, please? Tcl 8.7 is from 2007 so it seemed to me like ancient history not worth mentioning in code comments.

[StanFromIreland]

Tcl 8.7

Per the configure page, we still support building with it.

[jepler]

I rebased & removed the change to idlelib.

[StanFromIreland]

Please don't force push in the future, it makes reviewing harder.

[jepler]

Sorry, I'm still learning the norms here.