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-1001.0.46.4)] 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)
```