match CI's version of flake8 (#6941)

my flake8 wasn't up-to-date enough `make quality` wasn't reporting the same things CI did - this PR adds the actual required version.

Thinking more about some of these minimal versions - CI will always install afresh and thus will always run the latest version. Is there a way to tell pip to always install the latest versions of certain dependencies on `pip install -i ".[dev]"`, rather than hardcoding the minimals which quickly become outdated?

setup.py

@@ -92,7 +92,7 @@ extras["all"] = extras["serving"] + ["tensorflow", "torch"]
 extras["testing"] = ["pytest", "pytest-xdist", "timeout-decorator", "psutil", "parameterized"]
 # sphinx-rtd-theme==0.5.0 introduced big changes in the style.
 extras["docs"] = ["recommonmark", "sphinx", "sphinx-markdown-tables", "sphinx-rtd-theme==0.4.3", "sphinx-copybutton"]
-extras["quality"] = ["black >= 20.8b1", "isort >= 5", "flake8"]
+extras["quality"] = ["black >= 20.8b1", "isort >= 5", "flake8 >= 3.8.3"]
 extras["dev"] = extras["testing"] + extras["quality"] + extras["ja"] + ["scikit-learn", "tensorflow", "torch"]
 
 setup(