I wanted to specifically install libpython3.7-dev but it wasn't available in my default apt repositories.
My apt repositories had libpython3.8-dev which wouldn't work for my specific use-case.
I am on PopOS 20.04. So the corresponding debian codename would be "buster".
I noticed that it was available in https://debian.pkgs.org/10/debian-main-amd64/libpython3.7-dev_3.7.3-2+deb10u1_amd64.deb.html. But it's really hard to keep resolving dependencies.
I needed apt to fix it for me.
I noticed this ftp download URL in the above link: http://ftp.br.debian.org/debian/pool/main/p/python3.7/libpython3.7-dev_3.7.3-2+deb10u1_amd64.deb
So I added this to my /etc/apt/sources.list:
deb http://ftp.br.debian.org/debian/ buster main
(buster being the debian codename that matched with my PopOS 20.04)
I ran:
$ sudo apt update
but got these errors:
W: GPG error: http://ftp.br.debian.org/debian buster InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 04EE7237B7D453EC NO_PUBKEY 648ACFD622F3D138 NO_PUBKEY DCC9EFBF77E11517
E: The repository 'http://ftp.br.debian.org/debian buster InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
I then ran:
$ sudo apt-key adv --keyserver hkp://pool.sks-keyservers.net:80 --recv-keys 04EE7237B7D453EC
for each public key (replace 04EE7237B7D453EC with the next missing key and run again).
Then
$ sudo apt update
worked fine.
And yes, I am now able to install the originally missing libpython3.7-dev package now with apt.