Only install deps during tox, during CI (john-kurkowski#326)

tox installs deps itself. No need to do it first.

Saves some install time.

.github/workflows/ci.yml

@@ -41,8 +41,7 @@ jobs:
           check-latest: true
       - name: Install Python requirements
         run: |
-          pip install --upgrade pip
-          pip install --upgrade --editable '.[testing]'
+          pip install --upgrade tox
       - name: Test
         run: tox
         env: