I have the python3-tkinter installed, as shown below:
$ sudo yum install python3-tkinter [sudo] password for abigail: Loaded plugins: fastestmirror, langpacks L
For Python 3.6.x on CentOS 7.4, install:
Python 3.6.x
CentOS 7.4
$ sudo yum install python36-tkinter
You need to have EPEL repository configured.
EPEL