Suppress flake8's colored outputs

The outputs of flake8 are parsed by flycheck-parse-with-patterns, which does
not take ASCII escape sequences into account.
Therefore, flake8 should return simple texts (without escape sequences).

flycheck.el

@@ -10775,6 +10775,7 @@ Requires Flake8 3.0 or newer. See URL
   ;; Python versions; see https://github.com/flycheck/flycheck/issues/1055.
   :command ("python3"
             (eval (flycheck-python-module-args 'python-flake8 "flake8"))
+            "--color=never"
             "--format=default"
             (config-file "--append-config" flycheck-flake8rc)
             (option "--max-complexity" flycheck-flake8-maximum-complexity nil