Use editable pip install to fix testing of cython packages (#173)

In #172 the --no-build-isolation was added, needed to access to petsc
etc. packages during build of cython modules, but the -e flag was
dropped. It appears that the latter breaks importing a python module
with a cython submodule, from the source directory of that python module
itself.

E.g. if you are in the animate/ source directory, "import animate" in
python, it will try to import it from the local directory, but if that
module was built without the -e flag, the
numbering.cpython-312-x86_64-linux-gnu.so is not available in
animate/numbering/ but only in the installed directory
(/usr/local/lib/...) and therefore the import will fail on "from
.cython.numbering import to_petsc_local_numbering" Therefore you can no
longer run pytest within the animate/ directory.

With the -e flag, the dynamic .so is placed in animate/numbering/ and it
is able to "import animate" from within the local directory, as well as
from anywhere else.

Author: stephankramer

.github/workflows/reusable_test_suite.yml

@@ -45,7 +45,7 @@ jobs:
           git submodule init
           git submodule update
           pip uninstall ${REPO_NAME} -y || true
-          pip install --no-build-isolation .[dev]
+          pip install --no-build-isolation -e .[dev]
 
       - run: make lint