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.
13>C:\Program Files (x86)\MSBuild\Microsoft.Cpp\v4.0\Microsoft.MakeFile.Targets(38,5): error MSB3073: The command "
13>C:\Program Files (x86)\MSBuild\Microsoft.Cpp\v4.0\Microsoft.MakeFile.Targets(38,5): error MSB3073: IF EXIST ..\..\tcltk\lib\tix8.4.3\tix84g.dll (
13>C:\Program Files (x86)\MSBuild\Microsoft.Cpp\v4.0\Microsoft.MakeFile.Targets(38,5): error MSB3073:     echo Tix is already built and available.
13>C:\Program Files (x86)\MSBuild\Microsoft.Cpp\v4.0\Microsoft.MakeFile.Targets(38,5): error MSB3073:     exit /b 0
13>C:\Program Files (x86)\MSBuild\Microsoft.Cpp\v4.0\Microsoft.MakeFile.Targets(38,5): error MSB3073: )
13>C:\Program Files (x86)\MSBuild\Microsoft.Cpp\v4.0\Microsoft.MakeFile.Targets(38,5): error MSB3073: 
13>C:\Program Files (x86)\MSBuild\Microsoft.Cpp\v4.0\Microsoft.MakeFile.Targets(38,5): error MSB3073: IF NOT EXIST ..\..\tix-8.4.3.4 (
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.
13>C:\Program Files (x86)\MSBuild\Microsoft.Cpp\v4.0\Microsoft.MakeFile.Targets(38,5): error MSB3073:     exit 1
13>C:\Program Files (x86)\MSBuild\Microsoft.Cpp\v4.0\Microsoft.MakeFile.Targets(38,5): error MSB3073: )
13>C:\Program Files (x86)\MSBuild\Microsoft.Cpp\v4.0\Microsoft.MakeFile.Targets(38,5): error MSB3073: 
13>C:\Program Files (x86)\MSBuild\Microsoft.Cpp\v4.0\Microsoft.MakeFile.Targets(38,5): error MSB3073: IF "Win32" EQU "Win32" set TclMachine=IX86
13>C:\Program Files (x86)\MSBuild\Microsoft.Cpp\v4.0\Microsoft.MakeFile.Targets(38,5): error MSB3073: IF "Win32" EQU "x64" set TclMachine=AMD64
13>C:\Program Files (x86)\MSBuild\Microsoft.Cpp\v4.0\Microsoft.MakeFile.Targets(38,5): error MSB3073: 
13>C:\Program Files (x86)\MSBuild\Microsoft.Cpp\v4.0\Microsoft.MakeFile.Targets(38,5): error MSB3073: IF "Debug" EQU "Debug" (
13>C:\Program Files (x86)\MSBuild\Microsoft.Cpp\v4.0\Microsoft.MakeFile.Targets(38,5): error MSB3073:     set TixDebug=1
13>C:\Program Files (x86)\MSBuild\Microsoft.Cpp\v4.0\Microsoft.MakeFile.Targets(38,5): error MSB3073: ) ELSE (
13>C:\Program Files (x86)\MSBuild\Microsoft.Cpp\v4.0\Microsoft.MakeFile.Targets(38,5): error MSB3073:     set TixDebug=0
13>C:\Program Files (x86)\MSBuild\Microsoft.Cpp\v4.0\Microsoft.MakeFile.Targets(38,5): error MSB3073: )
13>C:\Program Files (x86)\MSBuild\Microsoft.Cpp\v4.0\Microsoft.MakeFile.Targets(38,5): error MSB3073: 
13>C:\Program Files (x86)\MSBuild\Microsoft.Cpp\v4.0\Microsoft.MakeFile.Targets(38,5): error MSB3073: cd ..\..\tix-8.4.3.4\win
13>C:\Program Files (x86)\MSBuild\Microsoft.Cpp\v4.0\Microsoft.MakeFile.Targets(38,5): error MSB3073: nmake -f python.mak MACHINE=%TclMachine% DEBUG=%TixDebug% TCL_DIR=F:\Python\dev\5\py35\PCbuild\..\..\tcl-8.6.1.0 TK_DIR=F:\Python\dev\5\py35\PCbuild\..\..\tk-8.6.1.0 all && nmake -f python.mak MACHINE=%TclMachine% DEBUG=%TixDebug% TCL_DIR=F:\Python\dev\5\py35\PCbuild\..\..\tcl-8.6.1.0 TK_DIR=F:\Python\dev\5\py35\PCbuild\..\..\tk-8.6.1.0 INSTALL_DIR=F:\Python\dev\5\py35\PCbuild\..\..\tcltk install
13>C:\Program Files (x86)\MSBuild\Microsoft.Cpp\v4.0\Microsoft.MakeFile.Targets(38,5): error MSB3073:     " exited with code 1.

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.