To install Python 3.9 locally, with ctypes, this worked for me.

# I created a temp directory
cd ~
mkdir tmp
cd tmp

# I downloaded and installed the latest libffi source from Github
wget "https://github.com/libffi/libffi/releases/download/v3.3/libffi-3.3.tar.gz"
tar xvf libffi-3.3.tar.gz
cd libffi-3.3
./configure --prefix=$HOME && make && make install

# I now had some libffi files in ~/lib/pkgconfig and ~/lib64.
# I added these to LD_LIBRARY_PATH, along with ~/lib.

export LD_LIBRARY_PATH="$HOME/lib/pkgconfig:$HOME/lib:$HOME/lib64:$LD_LIBRARY_PATH"

# I downloaded and installed the latest Python from python.org
cd ~/tmp
wget "https://www.python.org/ftp/python/3.9.0/Python-3.9.0.tgz"
cd ~/tmp/Python-3.9.0
./configure --prefix=$HOME --with-system-ffi LDFLAGS="-L $HOME/lib64/" CPPFLAGS="-I $HOME/tmp/libffi-3.3/include/"
make
make test
make install

# I was then able to run an installer that had shown me the error, "No module named '_ctypes'
pip3 install mycli

I hope this can be useful to you.

Raffi