tox.ini: generate HTML coverage output

This makes it trivial to inspect, line by line, what got covered.
That is useful when evaluating coverage for a new feature.

Signed-off-by: Martí Bolívar <mbolivar@amperecomputing.com>

tox.ini

@@ -37,6 +37,6 @@ setenv =
     # For instance: ./.tox/py3/tmp/
     TOXTEMPDIR={envtmpdir}
 commands =
-  python -m pytest --cov=west {posargs:tests} --basetemp='{envtmpdir}/pytest with space/'
+  python -m pytest --cov-report=html --cov=west {posargs:tests} --basetemp='{envtmpdir}/pytest with space/'
   python -m flake8 --config='{toxinidir}'/tox.ini '{toxinidir}'
   python -m mypy --config-file='{toxinidir}'/tox.ini --package=west