Cannot upgrade operating system of charm container

[Leon]

`juju refresh` does not support cross-release upgrades in k8s.

In an integration test I'm upgrading a charmhub charm (20.04) with a local charm (bumped charmcraft yaml to 22.04).
https://github.com/canonical/alertmanager-k8s-operator/blob/1ed7c1476e462dcabca9dac79db08f8c2e3a7675/tests/integration/test_upgrade_charm.py#L38

After upgrade the image is still 20.04:
```
$ k -n test-upgrade-charm-fhd8 describe pods/alertmanager-k8s-0
Containers:
  charm:
    Container ID: containerd://c41aa823521f9513a05d422916db1bb32ba5e5fac78a75dd6f4384b1aa03286c
    Image: jujusolutions/charm-base:ubuntu-20.04
    Image ID: docker.io/jujusolutions/charm-base@sha256:ce16c80092caef5439a3af0590bea671f9e85813051e37bafc8ebbb7ceaf8a21
```

And as a result I'm getting a new error that I do not see when deploying the 22.04 charm fresh:
```
13:55:47.095 WARNING unit.alertmanager-k8s/0.upgrade-charm ModuleNotFoundError: No module named '_cffi_backend'
13:55:47.095 WARNING unit.alertmanager-k8s/0.upgrade-charm thread '<unnamed>' panicked at 'Python API call failed', /github/home/.cargo/registry/src/index.crates.io-6f17d22bba15001f/pyo3-0.18.3/src/err/mod.rs:790:5
13:55:47.095 WARNING unit.alertmanager-k8s/0.upgrade-charm note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
13:55:47.095 WARNING unit.alertmanager-k8s/0.upgrade-charm Traceback (most recent call last):
13:55:47.095 WARNING unit.alertmanager-k8s/0.upgrade-charm File "./src/charm.py", line 29, in <module>
13:55:47.095 WARNING unit.alertmanager-k8s/0.upgrade-charm from charms.observability_libs.v0.cert_handler import CertHandler
13:55:47.095 WARNING unit.alertmanager-k8s/0.upgrade-charm File "/var/lib/juju/agents/unit-alertmanager-k8s-0/charm/lib/charms/observability_libs/v0/cert_handler.py", line 41, in <module>
13:55:47.095 WARNING unit.alertmanager-k8s/0.upgrade-charm from charms.tls_certificates_interface.v2.tls_certificates import ( # type: ignore
13:55:47.095 WARNING unit.alertmanager-k8s/0.upgrade-charm File "/var/lib/juju/agents/unit-alertmanager-k8s-0/charm/lib/charms/tls_certificates_interface/v2/tls_certificates.py", line 285, in <module>
13:55:47.095 WARNING unit.alertmanager-k8s/0.upgrade-charm from cryptography import x509
13:55:47.095 WARNING unit.alertmanager-k8s/0.upgrade-charm File "/var/lib/juju/agents/unit-alertmanager-k8s-0/charm/venv/cryptography/x509/__init__.py", line 7, in <module>
13:55:47.095 WARNING unit.alertmanager-k8s/0.upgrade-charm from cryptography.x509 import certificate_transparency
13:55:47.095 WARNING unit.alertmanager-k8s/0.upgrade-charm File "/var/lib/juju/agents/unit-alertmanager-k8s-0/charm/venv/cryptography/x509/certificate_transparency.py", line 11, in <module>
13:55:47.095 WARNING unit.alertmanager-k8s/0.upgrade-charm from cryptography.hazmat.bindings._rust import x509 as rust_x509
13:55:47.095 WARNING unit.alertmanager-k8s/0.upgrade-charm pyo3_runtime.PanicException: Python API call failed
13:55:47.306 ERROR juju.worker.uniter.operation hook "upgrade-charm" (via hook dispatching script: dispatch) failed: exit status 1
```

Seems like it's "impossible" to upgrade the operating system of a deployed charm.

Recently, PYDEPS encouraged us to switch from 20.04 to 22.04
https://github.com/canonical/charmcraft/issues/1192#issuecomment-1651919462

In the future there will probably be more and more reasons to want to run the charm container on a newer ubuntu.

[Jose C. Massón (jose-masson)]

I have found this issue as well, in loki-k8s:

unit-loki-k8s-0: 13:40:44.893 INFO juju.worker.uniter found queued "upgrade-charm" hook
unit-loki-k8s-0: 13:40:45.069 WARNING unit.loki-k8s/0.upgrade-charm ModuleNotFoundError: No module named '_cffi_backend'
unit-loki-k8s-0: 13:40:45.069 WARNING unit.loki-k8s/0.upgrade-charm thread '<unnamed>' panicked at 'Python API call failed', /github/home/.cargo/registry/src/index.crates.io-6f17d22bba15001f/pyo3-0.18.3/src/err/mod.rs:790:5
unit-loki-k8s-0: 13:40:45.069 WARNING unit.loki-k8s/0.upgrade-charm note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
unit-loki-k8s-0: 13:40:45.069 WARNING unit.loki-k8s/0.upgrade-charm Traceback (most recent call last):
unit-loki-k8s-0: 13:40:45.069 WARNING unit.loki-k8s/0.upgrade-charm File "./src/charm.py", line 33, in <module>
unit-loki-k8s-0: 13:40:45.069 WARNING unit.loki-k8s/0.upgrade-charm from charms.observability_libs.v0.cert_handler import CertHandler
unit-loki-k8s-0: 13:40:45.069 WARNING unit.loki-k8s/0.upgrade-charm File "/var/lib/juju/agents/unit-loki-k8s-0/charm/lib/charms/observability_libs/v0/cert_handler.py", line 41, in <module>
unit-loki-k8s-0: 13:40:45.069 WARNING unit.loki-k8s/0.upgrade-charm from charms.tls_certificates_interface.v2.tls_certificates import ( # type: ignore
unit-loki-k8s-0: 13:40:45.069 WARNING unit.loki-k8s/0.upgrade-charm File "/var/lib/juju/agents/unit-loki-k8s-0/charm/lib/charms/tls_certificates_interface/v2/tls_certificates.py", line 285, in <module>
unit-loki-k8s-0: 13:40:45.069 WARNING unit.loki-k8s/0.upgrade-charm from cryptography import x509
unit-loki-k8s-0: 13:40:45.069 WARNING unit.loki-k8s/0.upgrade-charm File "/var/lib/juju/agents/unit-loki-k8s-0/charm/venv/cryptography/x509/__init__.py", line 7, in <module>
unit-loki-k8s-0: 13:40:45.069 WARNING unit.loki-k8s/0.upgrade-charm from cryptography.x509 import certificate_transparency
unit-loki-k8s-0: 13:40:45.069 WARNING unit.loki-k8s/0.upgrade-charm File "/var/lib/juju/agents/unit-loki-k8s-0/charm/venv/cryptography/x509/certificate_transparency.py", line 11, in <module>
unit-loki-k8s-0: 13:40:45.069 WARNING unit.loki-k8s/0.upgrade-charm from cryptography.hazmat.bindings._rust import x509 as rust_x509
unit-loki-k8s-0: 13:40:45.069 WARNING unit.loki-k8s/0.upgrade-charm pyo3_runtime.PanicException: Python API call failed
unit-loki-k8s-0: 13:40:45.281 ERROR juju.worker.uniter.operation hook "upgrade-charm" (via hook dispatching script: dispatch) failed: exit status 1