Comment 1 for bug 1835738

I am not sure if this is the place to report this, but I cannot think of a better one to attract Mathias' attention, sorry.

Four python3.6 packages (and likely also python3.7 and python3.8) currently available in the ubuntu-toolchain-r PPA have an unmet version dependency on the 'python3' package. After adding the ppa, apt reports certain packages as upgradeable, but they cannot be upgraded:
--------------------------------
# lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 18.04.3 LTS
Release: 18.04
Codename: bionic
# uname -a
Linux [redacted] 4.15.0-64-generic #73-Ubuntu SMP Thu Sep 12 13:16:13 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux
# apt list --upgradable | grep python

WARNING: apt does not have a stable CLI interface. Use with caution in scripts.

libpython3.6/bionic 3.6.9-1~b1 amd64 [upgradable from: 3.6.8-1~18.04.2]
libpython3.6-dev/bionic 3.6.9-1~b1 amd64 [upgradable from: 3.6.8-1~18.04.2]
libpython3.6-minimal/bionic 3.6.9-1~b1 amd64 [upgradable from: 3.6.8-1~18.04.2]
libpython3.6-stdlib/bionic 3.6.9-1~b1 amd64 [upgradable from: 3.6.8-1~18.04.2]
python3-distutils/bionic 3.6.9-1~18.04 all [upgradable from: 3.6.8-1~18.04]
python3-gdbm/bionic 3.6.9-1~18.04 amd64 [upgradable from: 3.6.8-1~18.04]
python3-lib2to3/bionic 3.6.9-1~18.04 all [upgradable from: 3.6.8-1~18.04]
python3-tk/bionic 3.6.9-1~18.04 amd64 [upgradable from: 3.6.8-1~18.04]
python3.6/bionic 3.6.9-1~b1 amd64 [upgradable from: 3.6.8-1~18.04.2]
python3.6-dev/bionic 3.6.9-1~b1 amd64 [upgradable from: 3.6.8-1~18.04.2]
python3.6-minimal/bionic 3.6.9-1~b1 amd64 [upgradable from: 3.6.8-1~18.04.2]
python3.6-venv/bionic 3.6.9-1~b1 amd64 [upgradable from: 3.6.8-1~18.04.2]
# apt upgrade --dry-run libpython3.6 libpython3.6-dev libpython3.6-minimal libpython3.6-stdlib python3-distutils python3-gdbm python3-lib2to3 python3-tk python3.6-venv
Reading package lists... Done
Building dependency tree
Reading state information... Done
Calculating upgrade... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 python3-distutils : Depends: python3 (>= 3.6.8-1~) but 3.6.7-1~18.04 is to be installed
 python3-gdbm : Depends: python3 (>= 3.6.8-1~) but 3.6.7-1~18.04 is to be installed
 python3-lib2to3 : Depends: python3 (>= 3.6.8-1~) but 3.6.7-1~18.04 is to be installed
 python3-tk : Depends: python3 (>= 3.6.8-1~) but 3.6.7-1~18.04 is to be installed
E: Broken packages
--------------------------------

'apt show python3 -a' indicates the version 3.6.7-1~18.04 is from 'http://us.archive.ubuntu.com/ubuntu bionic-updates/main amd64', and there is also an original distro version 3.6.5-3 that hails from bionic/main. No other sources are available.