ci: pin coverage < 7 when running coveragepy-lcov (#33)

Pin coverage to < 7 when running coveragepy-lcov

coverage 7.0.0 removed `FnmatchMatcher`, which coverage-lcov
currently depends on.
chaychoong/coveragepy-lcov#5

We can unpin this once coverage-lcov has been fixed.

.github/workflows/coverage.yml

@@ -29,7 +29,7 @@ jobs:
       - name: Install tox
         run: |
           python -m pip install --upgrade pip
-          pip install tox coveragepy-lcov
+          pip install tox coveragepy-lcov 'coverage<7'
       - name: Run coverage
         run: tox -e coverage
       - name: Convert to lcov