calico-node service fails to start on xenial due to ASCII encoding

This appears to affect xenial only.

Calico units get stuck waiting/idle with status message: "Waiting to retry Calico node configuration". The units log an error from the calicoctl command:

2021-01-11 20:21:59 INFO juju-log b'null\nresource does not exist: Node(ip-172-30-0-150) with error: <nil>\n'

The node does not exist because calico-node fails to start:

Jan 11 20:24:36 ip-172-30-0-150 charm-env[26444]: Traceback (most recent call last):
Jan 11 20:24:36 ip-172-30-0-150 charm-env[26444]: File "/var/lib/juju/agents/unit-calico-0/.venv/bin/conctl", line 11, in <module>
Jan 11 20:24:36 ip-172-30-0-150 charm-env[26444]: load_entry_point('conctl-py35==0.1.2', 'console_scripts', 'conctl')()
Jan 11 20:24:36 ip-172-30-0-150 charm-env[26444]: File "/var/lib/juju/agents/unit-calico-0/.venv/lib/python3.5/site-packages/click/core.py", line 829, in __call__
Jan 11 20:24:36 ip-172-30-0-150 charm-env[26444]: return self.main(*args, **kwargs)
Jan 11 20:24:36 ip-172-30-0-150 charm-env[26444]: File "/var/lib/juju/agents/unit-calico-0/.venv/lib/python3.5/site-packages/click/core.py", line 760, in main
Jan 11 20:24:36 ip-172-30-0-150 charm-env[26444]: _verify_python3_env()
Jan 11 20:24:36 ip-172-30-0-150 charm-env[26444]: File "/var/lib/juju/agents/unit-calico-0/.venv/lib/python3.5/site-packages/click/_unicodefun.py", line 130, in _verify_python3_env
Jan 11 20:24:36 ip-172-30-0-150 charm-env[26444]: " mitigation steps.{}".format(extra)
Jan 11 20:24:36 ip-172-30-0-150 charm-env[26444]: RuntimeError: Click will abort further execution because Python 3 was configured to use ASCII as encoding for the environment. Consult https://click.palletsprojects.com/python3/ for mitigation steps.
Jan 11 20:24:36 ip-172-30-0-150 charm-env[26444]: This system supports the C.UTF-8 locale which is recommended. You might be able to resolve your issue by exporting the following environment variables:
Jan 11 20:24:36 ip-172-30-0-150 charm-env[26444]: export LC_ALL=C.UTF-8
Jan 11 20:24:36 ip-172-30-0-150 charm-env[26444]: export LANG=C.UTF-8
Jan 11 20:24:36 ip-172-30-0-150 systemd[1]: calico-node.service: Main process exited, code=exited, status=1/FAILURE