DevOps: Remove --use-feature flag in pip install of CI (aiidateam…

…#5703)

The `tests` job of the CI workflow used the `--use-feature=2020-resolver`
flag in the `pip install` call to force using a specific package
dependency resolver. We adopted this way back when `pip` introduced this
new dependency resolver to be early-adopters to detect problems as soon
as possible. In latest versions of `pip` this option is no longer
available since the resolver is now the default, so we simply remove the
flag.

.github/workflows/ci-code.yml

@@ -110,8 +110,8 @@ jobs:
 
     - name: Install aiida-core
       run: |
-        pip install --use-feature=2020-resolver -r requirements/requirements-py-${{ matrix.python-version }}.txt
-        pip install --use-feature=2020-resolver --no-deps -e .
+        pip install -r requirements/requirements-py-${{ matrix.python-version }}.txt
+        pip install --no-deps -e .
         pip freeze
 
     - name: Setup environment