Update pyflakes dependency from "git version" to 2.0.0

We can't remove the manual dependency completely yet, but it's better
to use a release version than the development version from the repo.

tox.ini

@@ -25,10 +25,11 @@ setenv =
 
 [testenv:flake8]
 basepython = python3.6
-# FIXME Remove the pyflakes git dependency once a new version of pyflakes (> 1.6.0) is released.
-# Currently we need to use the development version of pyflakes in order to use __class__, because pyflakes <= 1.6.0
-# doesn't understand __class__. This has been fixed in the development version, but not in any release yet.
+# FIXME Remove the manual pyflakes dependency once flake8 depends on pyflakes >= 2.0.0.
+# Currently we need to manually update pyflakes in order to use __class__, because pyflakes <= 1.6.0
+# doesn't understand __class__. This has been fixed in pyflakes 2.0.0, but flake8's requirements don't allow that
+# version of pyflakes yet.
 deps =
     flake8
-    git+https://github.com/PyCQA/pyflakes.git
+    pyflakes >= 2.0.0
 commands = flake8 {posargs}