As far as I know, the best information you can get from Tk about version is the patch level which does not necessarily take into account builds from a dev repo or patched source.  The easiest way to get that info from tkinter is to use test.pythoninfo:

$ python3.7 -m test.pythoninfo
[...]
tkinter.TCL_VERSION: 8.6
tkinter.TK_VERSION: 8.6
tkinter.info_patchlevel: 8.6.8
[...]

It may also be useful to ascertain which Tk libraries tkinter is linked with:

$ python3.7 -c 'import _tkinter;print(_tkinter.__file__)'
/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/lib-dynload/_tkinter.cpython-37m-darwin.so

$ otool -L $(python3.7 -c 'import _tkinter;print(_tkinter.__file__)')
/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/lib-dynload/_tkinter.cpython-37m-darwin.so:
	/Library/Frameworks/Python.framework/Versions/3.7/lib/libtcl8.6.dylib (compatibility version 8.6.0, current version 8.6.8)
	/Library/Frameworks/Python.framework/Versions/3.7/lib/libtk8.6.dylib (compatibility version 8.6.0, current version 8.6.8)
	/usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1197.1.1)