unpinning tenacity for python_version >='3.5' causes error for charms built with charms/2.x/stable

[Tianqi Xiao]

The recent changes introduced in MP#449280 [1] unpinned tenacity for python_version >= 3.5. However, the latest tenacity package is only compatible with python >= 3.7 [2], and it requires setuptools-scm >= 3.4 which contradict what is defined in layer-basic's wheelhouse.txt (setuptools-scm<=1.17.0;python_version < '3.8') [3]. This incompatibility causes `"install" hook error` for charms using this layer and built with `charms/2.x/stable`, which has core18 base and consequently uses python3.6.

Error message:
```
Collecting tenacity==8.2.2
  Installing build dependencies ... error
  Complete output from command /var/lib/juju/agents/unit-juju-local2-2/.venv/bin/python3 -m pip install --ignore-installed --no-user --prefix /tmp/pip-build-env-ogwnzt2c --no-warn-script-location --no-binary :none: --only-binary :none: --no-index --find-links wheelhouse -- "setuptools >= 21.0.0,!=24.0.0,!=34.0.0,!=34.0.1,!=34.0.2,!=34.0.3,!=34.1.0,!=34.1.1,!=34.2.0,!=34.3.0,!=34.3.1,!=34.3.2,!=36.2.0" setuptools_scm[toml]>=3.4:
  Looking in links: wheelhouse
  Collecting setuptools!=24.0.0,!=34.0.0,!=34.0.1,!=34.0.2,!=34.0.3,!=34.1.0,!=34.1.1,!=34.2.0,!=34.3.0,!=34.3.1,!=34.3.2,!=36.2.0,>=21.0.0
  Collecting setuptools_scm[toml]>=3.4
    Could not find a version that satisfies the requirement setuptools_scm[toml]>=3.4 (from versions: 1.17.0)
  No matching distribution found for setuptools_scm[toml]>=3.4
```

[1]: https://code.launchpad.net/~corey.bryant/layer-snap/+git/layer-snap/+merge/449280
[2]: https://pypi.org/project/tenacity/
[3]: https://github.com/juju-solutions/layer-basic/blob/master/wheelhouse.txt#L30