charm Install fails on Xenial

[Xav Paice]

When installing the charm on Xenial, the install hook fails with:

2021-07-20 05:53:42 WARNING unit.grafana/0.install logger.go:60 requests requires Python '>=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*, !=3.4.*, !=3.5.*' but the running Python is 3.5.2
2021-07-20 05:53:43 WARNING unit.grafana/0.install logger.go:60 Traceback (most recent call last):
2021-07-20 05:53:43 WARNING unit.grafana/0.install logger.go:60 File "/var/lib/juju/agents/unit-grafana-0/charm/hooks/install", line 8, in <module>
2021-07-20 05:53:43 WARNING unit.grafana/0.install logger.go:60 basic.bootstrap_charm_deps()
2021-07-20 05:53:43 WARNING unit.grafana/0.install logger.go:60 File "lib/charms/layer/basic.py", line 214, in bootstrap_charm_deps
2021-07-20 05:53:43 WARNING unit.grafana/0.install logger.go:60 env=_get_subprocess_env())
2021-07-20 05:53:43 WARNING unit.grafana/0.install logger.go:60 File "/usr/lib/python3.5/subprocess.py", line 581, in check_call
2021-07-20 05:53:43 WARNING unit.grafana/0.install logger.go:60 raise CalledProcessError(retcode, cmd)
2021-07-20 05:53:43 WARNING unit.grafana/0.install logger.go:60 subprocess.CalledProcessError: Command '['/var/lib/juju/agents/unit-grafana-0/.venv/bin/pip', 'install', '-U', '--force-reinstall', '--no-index', '--no-cache-dir', '-f', 'wheelhouse', 'requests==2.26.0', 'Tempita==0.5.2', 'PyYAML==5.2', 'charmhelpers==0.20.22', 'pyaml==20.4.0', 'six==1.16.0', 'certifi==2021.5.30', 'jsondiff==1.3.0', 'tenacity==5.0.3', 'urllib3==1.26.6', 'MarkupSafe==1.1.1', 'charset-normalizer==2.0.3', 'Jinja2==2.10.1', 'netaddr==0.7.19', 'idna==3.2', 'pbr==5.6.0', 'wheel==0.33.6', 'charms.reactive==1.4.1']' returned non-zero exit status 1
2021-07-20 05:53:43 ERROR juju.worker.uniter.operation runhook.go:139 hook "install" (via explicit, bespoke hook script) failed: exit status 1
2021-07-20 05:53:43 INFO juju.worker.uniter resolver.go:144 awaiting error resolution for "install" hook

Requests has not supported Python 3.5 after the release of 2.25