Specify coverage rule to combine results

We can specify a rule to map different source files from which coverage
stats were collected to the same source file in our repository.

.coveragerc

@@ -1,3 +1,11 @@
 [run]
 # this file comes from versioneer and we don't test it
 omit = */_version.py
+
+[paths]
+# This tells coverage how to combine results together or said differently
+# which files at different paths are actually the same file
+# documented at https://coverage.readthedocs.io/en/latest/config.html#paths
+source =
+    repo2docker/
+    /opt/hostedtoolcache/Python/*/site-packages/repo2docker

.github/workflows/test.yml

@@ -106,8 +106,7 @@ jobs:
           pip install --upgrade setuptools pip wheel
           pip install --upgrade -r dev-requirements.txt
           python setup.py bdist_wheel
-          #pip install dist/*.whl
-          pip install -e.
+          pip install dist/*.whl
           pip freeze
           # hg-evolve pinned to 9.2 because hg-evolve dropped support for
           # hg 4.5, installed with apt in Ubuntu 18.04