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 python3-pyqt5 python3-pyqt5.qsci
sudo apt install sip-py3api-12.7 python3-pyqt5 libqt5designer5 libqt5help5 libqt5test5 libqt5sql5

http://launchpadlibrarian.net/504110105/mercurial-common_5.5.2-1_all.deb
http://launchpadlibrarian.net/504110108/mercurial_5.5.2-1_amd64.deb
http://launchpadlibrarian.net/505758877/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

Note:
Debian SID now contains 5.6.1 which requires python3.9 which is not standard on Mint 20 and 6.1.1/6.1.2. Following for info only

sudo apt install python3.9
wget http://ftp.us.debian.org/debian/pool/main/m/mercurial/mercurial-common_5.6.1-4_all.deb
wget http://ftp.us.debian.org/debian/pool/main/m/mercurial/mercurial_5.6.1-4_amd64.deb
wget http://ftp.us.debian.org/debian/pool/main/t/tortoisehg/tortoisehg_5.6.1-1_all.deb

sudo dpkg -i mercurial-common_5.6.1-4_all.deb
sudo dpkg -i tortoisehg_5.6.1-1_all.deb
sudo dpkg -i mercurial_5.6.1-4_amd64.deb

or:
wget http://ftp.us.debian.org/debian/pool/main/m/mercurial/mercurial-common_6.1.2-1_all.deb
wget http://ftp.us.debian.org/debian/pool/main/m/mercurial/mercurial_6.1.2-1_amd64.deb
wget http://ftp.us.debian.org/debian/pool/main/t/tortoisehg/tortoisehg_6.1.1-1_all.deb