AttributeError: 'ConfigData' object has no attribute 'get' in Python 3.5.2 (Xenial)

It appears that typing.Mapping works a bit different in Python version 3.5.2 than other higher versions. I got the following error when running some functional tests on xenial, but it works on bionic.

Traceback (most recent call last):
  File "./src/charm.py", line 310, in <module>
    main(CloudstatsCharm)
  File "/var/lib/juju/agents/unit-cloudstats-0/charm/venv/ops/main.py", line 431, in main
    _emit_charm_event(charm, dispatcher.event_name)
  File "/var/lib/juju/agents/unit-cloudstats-0/charm/venv/ops/main.py", line 142, in _emit_charm_event
    event_to_emit.emit(*args, **kwargs)
  File "/var/lib/juju/agents/unit-cloudstats-0/charm/venv/ops/framework.py", line 316, in emit
    framework._emit(event)
  File "/var/lib/juju/agents/unit-cloudstats-0/charm/venv/ops/framework.py", line 784, in _emit
    self._reemit(event_path)
  File "/var/lib/juju/agents/unit-cloudstats-0/charm/venv/ops/framework.py", line 857, in _reemit
    custom_handler(event)
  File "./src/charm.py", line 145, in on_config_changed
    self.helper.configure()
  File "/var/lib/juju/agents/unit-cloudstats-0/charm/lib/lib_cloudstats.py", line 242, in configure
    self.render_config()
  File "/var/lib/juju/agents/unit-cloudstats-0/charm/lib/lib_cloudstats.py", line 118, in render_config
    context.update(self._openstack_context)
  File "/var/lib/juju/agents/unit-cloudstats-0/charm/lib/lib_cloudstats.py", line 146, in _openstack_context
    cert_config = self.charm_config.get("openstack_cert", None)
AttributeError: 'ConfigData' object has no attribute 'get'

Also see a simple code snippet

Python 3.5.2 (default, Jan 26 2021, 13:30:48) 
[GCC 5.4.0 20160609] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from typing import Mapping
>>> class foo(Mapping[str, str]):
...     def __init__(self):
...             self.data = {"greeting": "Hello World"}
...     def __len__(self):
...             return len(self.data)
...     def __iter__(self):
...             return iter(self.data)
...     def __getitem__(self, key: str) -> str:
...             return self.data[key]
... 
>>> f = foo()
>>> f.get("greeting")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'foo' object has no attribute 'get'
>>> f["greeting"]
'Hello World'

[benhoyt]

It’s very strange how “they” thought it was a good idea to change this in a patch version (between 3.5.2 and 3.5.3)!

[rwcarlsen]

Yeah - let’s cover all four.