[ci] Add Github action to check for unused imports

.github/workflows/main.yml

@@ -46,3 +46,25 @@ jobs:
     - name: Run Unittests
       run: |
         docker run reframe:${{ matrix.modules-version }}
+
+  flake8:
+    if: github.event_name == 'pull_request'
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+      with:
+        fetch-depth: 0
+    - name: Changed Python Files
+      run: |
+        echo 'CHANGED_PYTHON_FILES<<EOF' >> $GITHUB_ENV
+        git diff --name-only ${{ github.event.pull_request.base.sha }}..${{ github.sha }} | grep '.py' >> $GITHUB_ENV
+        echo 'EOF' >> $GITHUB_ENV
+    - uses: actions/setup-python@v2
+      with:
+        python-version: 3.8
+    - name: Unused Imports Check
+      run: |
+        pip install flake8
+        echo 'Checking Python Files:'
+        for f in "$CHANGED_PYTHON_FILES"; do echo "$f"; done
+        flake8 --select F401 $CHANGED_PYTHON_FILES

reframe/frontend/cli.py

@@ -16,7 +16,6 @@
 
 import reframe
 import reframe.core.config as config
-import reframe.core.environments as env
 import reframe.core.exceptions as errors
 import reframe.core.logging as logging
 import reframe.core.runtime as runtime

reframe/frontend/loader.py

@@ -10,12 +10,11 @@
 import ast
 import collections.abc
 import inspect
-import io
 import os
 
 import reframe.utility as util
 import reframe.utility.osext as osext
-from reframe.core.exceptions import NameConflictError, RegressionTestLoadError
+from reframe.core.exceptions import NameConflictError
 from reframe.core.logging import getlogger