No package in Mint20/Ubuntu20.04 (yet), need to install the debian versions of mercurial and tortoisehg.

TortoiseHg and Mercurial (+common) v5.5.2:
https://packages.debian.org/sid/all/mercurial-common
https://packages.debian.org/sid/amd64/mercurial
https://packages.debian.org/sid/tortoisehg

sudo apt install -y python3-iniparse

wget http://ftp.us.debian.org/debian/pool/main/m/mercurial/mercurial-common_5.5.2-1_all.deb
wget http://ftp.us.debian.org/debian/pool/main/m/mercurial/mercurial_5.5.2-1_amd64.deb
wget http://ftp.us.debian.org/debian/pool/main/t/tortoisehg/tortoisehg_5.5.2-1_all.deb

sudo dpkg -i mercurial-common_5.5.2-1_all.deb
sudo dpkg -i mercurial_5.5.2-1_amd64.deb
sudo dpkg -i tortoisehg_5.5.2-1_all.deb

#possibly required:
pip3 install pyqt5
pip3 install qscintilla
sudo apt install -y pyqt5.qsci-dev
sudo apt-get install python3-pyqt5.qsci