reshuffle responsibility for different parts of coverage handling

.circleci/report-coverage.sh

 #! /usr/bin/env nix-shell
 #! nix-shell -i bash -p "python.withPackages (ps: [ ps.codecov ])"
 find ./result-doc/share/doc
-cp ./result-doc/share/doc/*/.coverage.* ./
-python -m coverage combine
+cp ./result-doc/share/doc/*/.coverage ./
 python -m coverage report
 python -m coverage xml
 codecov --file coverage.xml

zkapauthorizer.nix

@@ -50,7 +50,7 @@ buildPythonPackage rec {
     runHook preCheck
     "${pyflakes}/bin/pyflakes" src/_zkapauthorizer
     ZKAPAUTHORIZER_HYPOTHESIS_PROFILE=${hypothesisProfile'} python -m ${if collectCoverage
-      then "coverage run --branch --source _zkapauthorizer,twisted.plugins.zkapauthorizer --module"
+      then "coverage run --debug=config --module"
       else ""
     } twisted.trial ${extraTrialArgs} ${testSuite'}
     runHook postCheck
@@ -58,6 +58,7 @@ buildPythonPackage rec {
 
   postCheck = if collectCoverage
     then ''
+    python -m coverage combine
     python -m coverage html
     mkdir -p "$doc/share/doc/${name}"
     cp -vr .coverage htmlcov "$doc/share/doc/${name}"