In my new installation with Buster, I faced the same problem and recreated "mlat-client_0.2.10_armhf.deb". After I ran dpkg -i, I copied the fa-mlat-client to /usr/bin and radarbox24 feeder picked it up and started MLAT. I then removed the packages build-essential debhelper python-dev python3-dev and the mlat-client folder.
But sudo apt-get update is again displaying the error:
The following packages have unmet dependencies:
mlat-client : Depends: python3 (< 3.6) but 3.7.3-1 is to be installed
E: Unable to correct problems, you have held broken packages.
The above packages have several dependencies which I did not want to keep in the system. I have kept git and curl though.
Its worth mentioning that mlat-client_0.2.10_armhf.deb package is also there under /home/pi/adsb-exchange and the sub folder adsb-exchange also contains another folder "mlat-client" but which may be a different version as judged by file size of fa-mlat-client:
1) /user/bin/fa-mlat-client * 2147bytes
2) /home/pi/adsb-exchange/mlat-client/fa-mlat-client * 2149 bytes