[gitlab-ci] Install pyflakes3 from debian testing

Otherwise we run into PyCQA/pyflakes#367

pyflakes setup.py rubber bin
156Traceback (most recent call last):
157  File "/usr/bin/pyflakes", line 11, in <module>
158    load_entry_point('pyflakes==2.0.0', 'console_scripts', 'pyflakes')()
159  File "/usr/lib/python3/dist-packages/pyflakes/api.py", line 201, in main
160    warnings = checkRecursive(args, reporter)
161  File "/usr/lib/python3/dist-packages/pyflakes/api.py", line 158, in checkRecursive
162    warnings += checkPath(sourcePath, reporter)
163  File "/usr/lib/python3/dist-packages/pyflakes/api.py", line 101, in checkPath
164    return check(codestr, filename, reporter)
165  File "/usr/lib/python3/dist-packages/pyflakes/api.py", line 73, in check
166    w = checker.Checker(tree, filename)
167  File "/usr/lib/python3/dist-packages/pyflakes/checker.py", line 497, in __init__
168    self.handleChildren(tree)
169  File "/usr/lib/python3/dist-packages/pyflakes/checker.py", line 824, in handleChildren
170    self.handleNode(node, tree)
171  File "/usr/lib/python3/dist-packages/pyflakes/checker.py", line 871, in handleNode
172    handler(node)
173  File "/usr/lib/python3/dist-packages/pyflakes/checker.py", line 824, in handleChildren
174    self.handleNode(node, tree)
175  File "/usr/lib/python3/dist-packages/pyflakes/checker.py", line 871, in handleNode
176    handler(node)
177  File "/usr/lib/python3/dist-packages/pyflakes/checker.py", line 1306, in TUPLE
178    self.handleChildren(node)
179  File "/usr/lib/python3/dist-packages/pyflakes/checker.py", line 824, in handleChildren
180    self.handleNode(node, tree)
181  File "/usr/lib/python3/dist-packages/pyflakes/checker.py", line 871, in handleNode
182    handler(node)
183  File "/usr/lib/python3/dist-packages/pyflakes/checker.py", line 824, in handleChildren
184    self.handleNode(node, tree)
185  File "/usr/lib/python3/dist-packages/pyflakes/checker.py", line 870, in handleNode
186    handler = self.getNodeHandler(node.__class__)
187  File "/usr/lib/python3/dist-packages/pyflakes/checker.py", line 700, in getNodeHandler
188    self._nodeHandlers[node_class] = handler = getattr(self, nodeType)
189AttributeError: 'Checker' object has no attribute 'CONSTANT'
190make: *** [Makefile:16: pyflakes] Error 1

.gitlab-ci.yml

@@ -18,4 +18,8 @@ test:
       ./run.sh --debchroot *
 
 lint:
-  script: make lint
+  script:
+    - apt-get install -y -t testing python3-pyflakes
+    - rm -f /usr/bin/pyflakes
+    - ln /usr/bin/pyflakes3 /usr/bin/pyflakes
+    - make lint