After updating the hw-health charm revision 24 to 30 on an Ubuntu bionic machine, the upgrade failed because the charm doesn't work with python 3.6 anymore, which is the default python version for Ubuntu bionic.
Here is the traceback from `juju debug-log`:
```bash
unit-hw-health-38: 16:55:22 WARNING unit.hw-health/38.upgrade-charm pip requires Python '>=3.7' but the running Python is 3.6.9
unit-hw-health-38: 16:55:22 WARNING unit.hw-health/38.upgrade-charm Traceback (most recent call last):
unit-hw-health-38: 16:55:22 WARNING unit.hw-health/38.upgrade-charm File "/var/lib/juju/agents/unit-hw-health-38/charm/hooks/upgrade-charm", line 8, in <module>
unit-hw-health-38: 16:55:22 WARNING unit.hw-health/38.upgrade-charm basic.bootstrap_charm_deps()
unit-hw-health-38: 16:55:22 WARNING unit.hw-health/38.upgrade-charm File "lib/charms/layer/basic.py", line 202, in bootstrap_charm_deps
unit-hw-health-38: 16:55:22 WARNING unit.hw-health/38.upgrade-charm _update_if_newer(pip, pre_install_pkgs)
unit-hw-health-38: 16:55:22 WARNING unit.hw-health/38.upgrade-charm File "lib/charms/layer/basic.py", line 367, in _update_if_newer
unit-hw-health-38: 16:55:22 WARNING unit.hw-health/38.upgrade-charm pkg], env=_get_subprocess_env())
unit-hw-health-38: 16:55:22 WARNING unit.hw-health/38.upgrade-charm File "/usr/lib/python3.6/subprocess.py", line 311, in check_call
unit-hw-health-38: 16:55:22 WARNING unit.hw-health/38.upgrade-charm raise CalledProcessError(retcode, cmd)
unit-hw-health-38: 16:55:22 WARNING unit.hw-health/38.upgrade-charm subprocess.CalledProcessError: Command '['/var/lib/juju/agents/unit-hw-health-38/.venv/bin/pip', 'install', '-U', '--no-index', '-f', 'wheelhouse', 'pip']' returned non-zero exit status 1.
unit-hw-health-38: 16:55:22 ERROR juju.worker.uniter.operation hook "upgrade-charm" (via explicit, bespoke hook script) failed: exit status 1
```
The charm supposedly supports Ubuntu bionic, but the Python version running in bionic by default (`3.6.9`) doesn't work with the charm.
Local test environment:
1. Deploy ubuntu bionic series.
$ juju deploy ubuntu --series bionic ubuntu-bionic
2. Deploy hw-health rev 24 and rev 30 (rev 30 is from latest/stable channel)
$ juju deploy hw-health --revision 24 --channel latest/stable hw-health-old
$ juju deploy hw-health
Upon trying to reproduce the issue, these are my findings:
1. When trying to relate hw-health rev 30 charm with ubuntu-bionic, I'm facing the same error as presented in the bug report.
2. When trying to relate the hw-health rev 24 charm with ubuntu-bionic, the hw-health charm is stuck in `maintenance` phase with the message "Installing freeipmi,
We could possibly deprecate bionic (along with xenial 16.04) since bionic is already EOL and there doesn't seem to be an easy way to fix the python version dependency for pip (the error appears from the lower level install/upgrade hook code).