Stop using tox for retworkx backwards compat jobs

Tox's isolated builder mechanism seems to be incompatible with our
environment variable based package naming mechanism that we use to build
the legacy retworkx package. This is causing CI to fail on the backwards
compat jobs that are installing retworkx (which depends on rustworkx) to
ensure that our backwards compatibility shim works as expected. Instead
of trying to force tox to do the correct thing, it's just easier to stop
using it for that one CI job and instead just manually install and run
the tests.

.github/workflows/main.yml

@@ -131,18 +131,18 @@ jobs:
         with:
           toolchain: ${{ matrix.rust }}
           targets: ${{ matrix.platform.rust-target }}
-      - name: 'Install dependencies'
-        run: python -m pip install --upgrade tox
       - name: 'Install binary dependencies'
         run: sudo apt-get install -y graphviz
         if: runner.os == 'Linux'
-      - name: 'Build rustworkx'
-        run: |
-          tox -epy --notest
-      - name: 'Run retworkx tests'
+      - name: 'Build rustworkx and test dependencies'
         env:
           RUSTWORKX_PKG_NAME: "retworkx"
-        run: tox -epy -- -t ./retworkx_backwards_compat
+        run: |
+          pip install -c constraints.txt -U '.[mpl,graphviz]' fixtures testtools>=2.5.0 networkx>=2.5 stestr
+      - name: 'Run retworkx tests'
+        run: |
+          cd tests
+          stestr run -t ./retworkx_backwards_compat
   coverage:
     needs: [tests]
     name: Coverage