simplify and document how to measure code coverage

This patch configures Coverage.py to measure only the code coverage of
kas itself. In addition, we add a note in the devguide how to create and
inspect the coverage data.

Signed-off-by: Felix Moessbauer <felix.moessbauer@siemens.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

docs/devguide.rst

@@ -32,6 +32,18 @@ When you need a proxy to access the internet, add::
 to the call.
 
 
+Measure code coverage
+---------------------
+
+To measure the code coverage of the unit tests, the ``pytest-cov`` package is
+required. On Debian systems, this is provided in ``python3-pytest-cov``.
+Once installed, run::
+
+    $ python3 -m pytest --cov --cov-report html
+
+The coverage in HTML format can then be found in `htmlcov`.
+
+
 Community Resources
 -------------------