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Same behaviour experienced here on Mojave Kernel Version 18.6.0.
Homebrew Python 3.7.3
Python 3.7.3 (default, Jun 19 2019, 07:38:49) 
[Clang 10.0.1 (clang-1001046.4)] on darwin
Type "help", "copyright", "credits" or "license()" for more information.
WARNING: The version of Tcl/Tk (8.5.9) in use may be unstable. Visit
http://www.python.org/download/mac/tcltk/ for current information.
>>> 
It seems to be compiled using Tcl 8.5 that is installed with the system.
IDLE shipped with Anaconda works correctly.
```
brew linkage python
System libraries:
 /System/Library/Frameworks/AppKit.framework/Versions/C/AppKit
 /System/Library/Frameworks/Carbon.framework/Versions/A/Carbon
 /System/Library/Frameworks/CoreFoundation.framework/Versions/A/CoreFoundation
 /System/Library/Frameworks/CoreServices.framework/Versions/A/CoreServices
 /System/Library/Frameworks/Foundation.framework/Versions/C/Foundation
 /System/Library/Frameworks/SystemConfiguration.framework/Versions/A/SystemConfiguration
 /System/Library/Frameworks/Tcl.framework/Versions/8.5/Tcl
 /System/Library/Frameworks/Tk.framework/Versions/8.5/Tk
 /usr/lib/libSystem.B.dylib
 /usr/lib/libbz2.1.0.dylib
 /usr/lib/libncurses.5.4.dylib
 /usr/lib/libobjc.A.dylib
 /usr/lib/libpanel.5.4.dylib
 /usr/lib/libz.1.dylib
Homebrew libraries:
 /usr/local/opt/gdbm/lib/libgdbm.6.dylib (gdbm)
 /usr/local/opt/openssl/lib/libcrypto.1.0.0.dylib (openssl)
 /usr/local/opt/openssl/lib/libssl.1.0.0.dylib (openssl)
 /usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/Python (python)
 /usr/local/opt/readline/lib/libreadline.8.dylib (readline)
 /usr/local/opt/sqlite/lib/libsqlite3.0.dylib (sqlite)
 /usr/local/opt/xz/lib/liblzma.5.dylib (xz)
```