Problem has been solved after installation of ActiveTcl 8.5.18.0 from ActiveState's web site as per Terry’s recommendations and, then, reinstallation of Python 3.6.2 package from python.org. However, there were many (mis)steps per my ignorance and laziness in between. Thank you very much for all the help!

Noteworthy case notes:

1. The previously installed IDLE 3.6.2 as part of the Python 3.6.2 package continued crashing whenever I entered the caret character after the installation of ActiveTcl 8.5.18.0. I noticed that the IDLE’s Tk version showed 8.5.9 even after I installed, as recommended, ActiveTcl 8.5.18.0, and rebooted my computer. Note that the “info patchlevel” script I executed within tclsh had started returning “8.5.18” instead of “8.5.9” after the installation of ActiveTcl 8.5.18.0.

2. I could not find a way to make the existing Python / IDLE 3.6.2 installation recognise ActiveTcl 8.5.18.0 as the IDLE’s Tk version kept showing 8.5.9. Therefore, I  reinstalled the Python 3.6.2 package downloaded from python.org. The package made the installation to a new location rather than overwriting the existing Python / IDLE 3.6.2 files.

3. The newly installed IDLE 3.6.2 has been working fine without showing any signs of instability. There are no crashes when I type the caret character. Its Tk version shows 8.5.18 along with the tclsh interpreter. 

4. I removed all the files associated with the “buggy” Python / IDLE 3.6.2 installation via the terminal window in order to prevent any future confusion between the old and new installations.

5. Though I installed ActiveTcl 8.6.6.8606 from ActiveState’s web site in my earlier trials, I don’t think I was able to fully test whether this latest Tcl really works with Python / IDLE 3.6.2 or not. After installing ActiveTcl 8.6.6.8606, the “info patchlevel” I executed within tchlsh started returning “8.6.6” but the (old) IDLE 3.6.2’s Tk version still showed 8.5.9. I, then, installed ActiveTcl 8.5.18.0 after uninstalling ActiveTcl 8.6.6.8606 as a precaution. It  occurred to me that I may need to reinstall the Python 3.6.2 package from python.org only after installing the ActiveTcl 8.5.18.0.