Move test-only packages into requirements_testing.txt

CI installed coverage, tblib and dulwich ad hoc and unpinned. Collect them
in a pinned requirements_testing.txt instead, matching how requirements.txt
declares runtime dependencies.

The file is not added to the sdist include list, since test packages are
not needed to build or install NetBox, and verify_dependencies.py continues
to compare only base_requirements.txt against requirements.txt.

.github/workflows/ci.yml

@@ -121,11 +121,7 @@ jobs:
         run: |
           python -m pip install --upgrade pip
           pip install -r requirements.txt
-          pip install coverage tblib
-          # dulwich is the optional 'git' extra, so it is absent from requirements.txt.
-          # The Git data backend tests mock dulwich.porcelain.clone and only need the
-          # module to be importable; without it they skip silently.
-          pip install dulwich
+          pip install -r requirements_testing.txt
 
       - name: Check for missing migrations
         run: python netbox/manage.py makemigrations --check

requirements_testing.txt

@@ -0,0 +1,5 @@
+# Packages needed to run the test suite, but not to run NetBox itself.
+# dulwich is the optional 'git' extra, so it is absent from requirements.txt.
+coverage==7.16.0
+dulwich==1.2.14
+tblib==3.2.2