System info:
macOS 10.14.6 (Mojave), tcl-tk 8.6.9, pyenv 1.2.15, and python 3.8.0.

ModuleNotFoundError: No module named '_tkinter'

1. Install pyenv via homebrew
   brew install pyenv
2. Install tcl-tk via homebrew
   brew install tcl-tk
3. Add environment variables into ~/.zshrc

python pyenv

export PATH="PATH"
if which pyenv > /dev/null; then
eval "$(pyenv init -)";
fi

pyenv-virtualenv

if which pyenv-virtualenv-init > /dev/null; then
eval "$(pyenv virtualenv-init -)";
fi

tcl-tk

export PATH="/usr/local/opt/tcl-tk/bin:$PATH"
export LDFLAGS="-L/usr/local/opt/tcl-tk/lib"
export CPPFLAGS="-I/usr/local/opt/tcl-tk/include"
export PKG_CONFIG_PATH="/usr/local/opt/tcl-tk/lib/pkgconfig"
export PYTHON_CONFIGURE_OPTS="--with-tcltk-includes='-I/usr/local/opt/tcl-tk/include' --with-tcltk-libs='-L/usr/local/opt/tcl-tk/lib -ltcl8.6 -ltk8.6'"

4. Install pyenv-virtualenv via homebrew
   brew install pyenv-virtualenv
5. Install python by pyenv (3.8.0 for example)
   pyenv install 3.8.0
6. Restart shell
   exec $SHELL
7. Create a local environment of python 3.8.0 by pyenv:
   pyenv local 3.8.0
   pyenv version
8. Test tcl-tk installation with:
   python -m tkinter -c 'tkinter._test()'
   or simply:
   idle
   Job done.

references:
https://github.com/pyenv/pyenv/issues/1375#issuecomment-524280004
https://github.com/pyenv/pyenv/issues/1375#issuecomment-549754431