Use a non-hidden filename for coverage files

The `upload-artifact` action recently introduced a change where by default it no
longer uploads hidden files. This affected the `.coverage.*` files produced by
Python tests. There is an `include-hidden-files: true` option to change this,
but so far in our testing it did not work. Fix the issue by changing the file
name to be non-hidden.

See actions/upload-artifact#602 and
https://hypothes-is.slack.com/archives/C4K6M7P5E/p1725285976308269.

.github/workflows/ci.yml

@@ -95,12 +95,14 @@ jobs:
       - run: python -m pip install 'tox<4'
       - run: tox -e tests
         env:
-          COVERAGE_FILE: .coverage.${{ matrix.python-version }}
+          # Use a non-hidden file name to work around issues uploading hidden
+          # files in `actions/upload-artifact`.
+          COVERAGE_FILE: coverage.tests
       - name: Upload coverage file
         uses: actions/upload-artifact@v3
         with:
           name: coverage
-          path: .coverage.*
+          path: coverage.tests
   Coverage:
     needs: tests
     runs-on: ubuntu-latest
@@ -123,6 +125,10 @@ jobs:
           name: coverage
       - run: python -m pip install 'tox<4'
       - run: tox -e coverage
+        env:
+          # nb. In this context `COVERAGE_FILE` specifies a basename. All files
+          # matching `coverage.{name}` will be combined.
+          COVERAGE_FILE: coverage
   Functests:
     runs-on: ubuntu-latest
     services: