Use setup.py develop for now

Some change in `setuptools` or `pip` seems to have dropped the ability
to retain the build directory, which we need for generating coverage of
the .c files. I've spent a bit of time trying to make this work with
`pip install -e .` and can't seem to get things to work. We'll use the
deprecated command for now.

.github/workflows/ci.yml

@@ -46,7 +46,9 @@ jobs:
           MSGSPEC_COVERAGE: "true"
         run: |
           python setup.py clean --all
-          pip install -e .
+          # I know this is deprecated, but I can't find a way to keep the build
+          # directory around anymore on new versions of setuptools
+          python setup.py develop
 
       - name: Run tests with sanitizers
         env: