Try enabling coverage parallel feature so combine works better

We're abusing combine to rewrite paths in the coverage data.  combine isn't
intended to be used with only one coverage data file named ".coverage".  So
enable parallel mode so we have only one coverage data file with the naming
scheme "combine" expects.

Maybe this gets us working path rewriting on Windows,

.circleci/report-coverage.sh

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

.coveragerc

@@ -10,3 +10,4 @@ source =
     C:\hostedtoolcache\windows\Python\2.7.18\x64\Lib\site-packages\
 
 branch = True
+parallel = True

.github/workflows/ci.yaml

@@ -70,7 +70,7 @@ jobs:
         echo "Files:"
         dir
         echo "Combining"
-        coverage combine .coverage
+        coverage combine
         echo "Reporting"
         coverage report
         echo "Converting to XML"