Terry J. Reedy added the comment:
> I just pulled and recompiled and got makefile error messages that I do not remember seeing:
> 13>EXEC : error : ..\..\tix-8.4.3.4 doesn't exist.

This is the output of this line:
<snip 7 lines>

> 13>C:\Program Files (x86)\MSBuild\Microsoft.Cpp\v4.0\Microsoft.MakeFile.Targets(38,5): error MSB3073:     echo error: ..\..\tix-8.4.3.4 doesn't exist.

The rest of the messages are due to the following line:

> 13>C:\Program Files (x86)\MSBuild\Microsoft.Cpp\v4.0\Microsoft.MakeFile.Targets(38,5): error MSB3073:     exit 1

MSbuild just feels the need to output the entire command when it gets
a failure at any point in it.

The "error: ..\..\tix-8.4.3.4 doesn't exist" implies that you haven't
run Tools/buildbot/external-common.bat since you updated past
8cf384852680.  Tix has been included in the Windows installer for
quite some time, but had never been pulled in by the external*.bat
scripts or mentioned in the build process at all until this issue.  As
far as I know, Tix is not tested at all currently and nothing in the
source tree depends on it.

> I also got 6 failures instead of the usual 3 or 4.
>
> On the other hand, import idlelib.idle worked and the test_idle work both alone and when running everything.

test_idle will most likely fail if you run the tkinter tests first
(i.e., PCbuild\python_d.exe -m test -uall test_tcl test_tk
test_ttk_guionly test_ttk_textonly test_idle).  Issue #20035 should
fix that (but still needs review).