Fix CI issue after flake8 update 5 → 6

PyCQA/flake8#1760

Plugin flake8-quotes compatible with flake8 6.0 starting with 3.3.2:
zheller/flake8-quotes#111 is merged.

tox.ini

@@ -13,7 +13,7 @@ setenv=
 
 [testenv:flake8]
 skip_install = true
-deps = flake8<6.0
+deps = flake8
        flake8-author
        flake8-blind-except
        flake8-bugbear
@@ -24,7 +24,7 @@ deps = flake8<6.0
        flake8-exact-pin
        flake8-isort
        flake8-print
-       flake8-quotes
+       flake8-quotes>=3.3.2  # for compatibility with flake8 6.0
        flake8-tidy-imports
        flake8-tuple
        pep8-naming