Use also Flake8

Author: aaltat

Diff for: .flake8

@@ -0,0 +1,4 @@
+[flake8]
+exclude =
+   __pycache__,
+ignore = E501

Diff for: CONTRIBUTING.rst

@@ -107,11 +107,22 @@ needed in internal code. When docstrings are added, they should follow
 `PEP-257`_. See `Documentation`_ section below for more details about
 documentation syntax, generating docs, etc.
 
-The code should be formatted with `Black`_ and black can be run by using
+The code should be formatted with `Black`_ and errors found by `flake8`_
+should be fixed. Black and flake8 can be run by using
 command::
 
     inv lint
 
+By default flake8 ignores line length error E501, but it does not ignore
+warning W503. In practice Black formats list access like this::
+
+    list[1 : 2]
+
+But flake8 will display an warning about it. This should be manually
+fixed to look like::
+
+    list[1:2]
+
 Documentation
 -------------
 
@@ -215,4 +226,5 @@ the same code as your changes. In that case you should
 .. _test/README.rst`: https://github.com/robotframework/SeleniumLibrary/blob/master/test/README.rst
 .. _sync your fork: https://help.github.com/articles/syncing-a-fork/
 .. _resolve conflicts: https://help.github.com/articles/resolving-a-merge-conflict-from-the-command-line
-.. _Black: https://github.com/psf/black
+.. _Black: https://github.com/psf/black
+.. _flake8: https://gitlab.com/pycqa/flake8

Diff for: tasks.py

@@ -151,3 +151,4 @@ def init_labels(ctx, username=None, password=None):
 @task
 def lint(ctx):
     ctx.run("black --config pyproject.toml src/ utest/ atest/")
+    ctx.run("flake8 --config .flake8 src/ utest/ atest/")