Force requests>=2.4.2 via wheelhouse.txt

[Alvaro Uria]

* requests python lib supports "json" argument from v2.4.2 onwards [1].
* grafana-charm uses the "json" arg but doesn't force a python3-requests version higher than v2.4.2 [2]
* Since the charm uses layer-basic [3], I think it should use wheelhouse.txt to enforce such versions [4]

1. https://github.com/requests/requests/commit/8f17741849edb5e7eba0356f90cb17e43c938a2f
2. https://git.launchpad.net/grafana-charm/tree/reactive/grafana.py?id=ae04c7dee2198cb1fcc3544096cd47900256da41#n546
3. https://git.launchpad.net/grafana-charm/tree/layer.yaml?id=ae04c7dee2198cb1fcc3544096cd47900256da41
4. https://charmsreactive.readthedocs.io/en/latest/layer-basic.html

wheelhouse.txt:
"""
requests>=2.4.2
"""

When I upgraded the lib manually, I also had to upgrade urllib3.
"""
cd /var/lib/juju/agents/unit-grafana-0/.venv/
. bin/activate
pip install --upgrade pip
pip3 install requests --upgrade
deactivate
cd
restart jujud-unit-grafana-0
"""

[Alvaro Uria (aluria)]

I saw this issue in a trusty environment, so it's likely that newer Ubuntu series won't hit this bug.